Is there a scale or classification for scholars and their qualification of hadith narrators?

A source and a "how to"

To answer this one needs to be read, search and investigate in all available sources if Jarh and Ta'adil.

Nevertheless there's at least one book that I've learnt about that gives an attempt to answer this. It is a longer paper or letter called:
_{Important note:
In the following and later I'll be translating from Arabic. As these translations are of my own take them with the necessary care.
Anything written in brackets or superscript is either an -explanatory- addition (which not necessarily appears in the original text) or a synonym or explanation of mine}

ذكر من يعتمد قوله في الجرح والتعديل

Transliteration: dhikr man yu'tmadu qawluhu fi l-Jarhe wa-T'adeele
Translation: Presentation of those whoms views are counted on in Jarh and Ta'adeel

(See here page 169 ff)

of imam a-Dhahabi. Which starts with the following words:

بسم الله الرحمن الرحيم اعلم هداك الله أن الذين قبل الناس قولهم في الجرح والتعديل على ثلاث أقسام
In the name of Allah , the Entirely Merciful, the Especially Merciful, be aware may Allah guide you that those whom people (of knowledge) accepted their qualification in Jarh and Ta'adil are of three categories:

• قسم تكلموا في أكثر الرواة كابن معين وأبي حاتم الرازي.
  A category of people who have quoted (addressed/qualified) most (many/more than all the others) narrators like:
  Ibn Ma'yn and Abu Hatim ar-Razi.

• وقسم تكلموا في كثير من الرواة كمالك وشعبة. And a category of people who have quoted many narrators like:
  Malik and Sho'abah.

• وقسم تكلموا في الرجل بعد الرجل كابن عيينة والشافعي
  And a category of people who have quoted a person after the other like:
  ibn 'Oyynah and a-Shafi'i.

والكل أيضا على ثلاث أقسام.
And the whole can be divided into three categories too:

1. قسم منهم في الجرح متثبت في التعديل يغمز الراوي بالغلطتين والثلاث ويلين بذلك حديثه:
   A first category among them who was very strict in his Jarh ("accusation") and who may wink in his Ta'adil at two or three mistakes and this way tempers his hadith:
   فهذا اذا وثق شخصا فعض على قوله بناجذيك وتمسك بتوثيقه وإذا ضعف رجلا فانظر هل وافقه غيره على تضعيفه إن وافقه ولم يوثق ذلك أحد من الحذاق فهو ضعيف وإن وثقه أحد فهذا الذي قالوا فيه لا يقبل تجرحه إلا مفسرا يعني لا يكفي أن يقول فيه ابن معين مثلا هو ضعيف ولم يوضح سبب ضعفه وغيره قد وثقه فمثل هذا يتوقف في تصحيح حديثه وهو إلى الحسن اقرب وابن معين وأبو حاتم والجوزجاني متعنتون.
   One of those if he declared a person as trustworthy, then take it as it is and trust his qualification. And if he declared a person as weak, then check whether others have given an equivalent qualification about this weakness. As if they agreed with him and none among those well versed declared this person trustworthy, then this person is weak. But if any of those disagreed and declared this person trustworthy, then this qualification is only valid with a proof (explanation). This means for example if ibn Ma'yn said this person is da'if and didn't make clear why he declared him da'if, while others declared that person trustworthy, then such a person might not be declared as "sahih in his hadith transmission" (highest degree of trust), but he might be closer to "hassan" (high degree of trust).
   Note that ibn Ma'yn, abu Hatim and al-Juzajani are hard-liners in this.

2. وقسم في مقابلة هؤلاء كأبي عيسى الترمذي وأبي عبد الله الحاكم وأبي بكر البيهقي متساهلون.
   And on the opposite of the category of those there are:
   Abu 'isa a-Thirmidi, abu 'Abdullah al_Hakim and abu Bakr al-Bayhaqi which are (very) lenient.

3. وقسم كالبخاري واحمد بن حنبل وأبي زرعة وابن عدي معتدلون ومنصفون.
   And finally there's the category of scholars such as:
   Al-Bukhari, Ahamd ibn Hanbal, Abu Zur'ah, ibn 'Ady who are more moderate and fair

After that a-Dhahabi started citing names of the earliest scholars who have practiced Jarh and Ta'adil before moving to cite them in a tabaqaat manner.

Note that this description generally works for narrators who have narrated many ahadith. As for those who have only transmitted a handful of narrations the matter is different. Further note that even a scholar classified above or in the upcoming list as lenient could appear as a hardliner and vice versa. For example imam Malik had a great knowledge about scholars and narrators from al-Hijaz, while he generally was careful in accepting a narrator from al-'Iraq and considered them as rather weak unless shown otherwise. Nevertheless he made a mistake when accepting ibn al-Mukahriq who appeared as a truthful person.

A longer adapted list of a contemporary scholar

Here a longer extract made by a contemporary scholar called ibn Amin which is a bit different classification which is based on imam a-Dhahabi's letter and other sources such as al-Muqidah الموقظة, ar-Rad al-Wafir الرد الوافر of ibn Nasiruddin and ibn Hajars work An-Nakt النكت etc.:

Hardliners المتشددون:

• Abu Hatim ar-Razi أبو حاتم الرازي,
• Yahya ibn Sa'id al-Qattanيحيى بن سعيد القطان ,
• Al-'Uqayli العُقَيْلي ,
• Abu al-Fath al-Azdyأبو الفتح الأزدي ,
• Ibn Hebban ^{(in his book al-Majruhyn)} ابن حِبَّان ^{(في كتابه المجروحين)} ,
• Abu Nu'aym ibn Dukkayn al-Mula'y أبو نعيم الفضل بن دكين المُلائي,
• 'Affan ibn Muslim a-Saffar عفان بن مسلم الصَفّار,
• ibn Kharrash ^{He was declared as rafidy by the author of my source, a-Dhahabi only quoted that he had shi'a tendency's in his siyar a'alam an-Nubala'} ابن خراش.

The rather hardliners between the moderates المعتدلون مع بعض التشدد:

• Imam al-Bukhari البُخاري,
• imam Muslim مُسلم,
• Yahya ibn Ma'yn يحيى بن معين,
• Abu Zur'a ar-Razi أبو زُرعة الرازي,
• imam an-Nasa-i النَّسائي,
• 'Ali ibn al-Madyny علي بن المَديْني,
• imam Malik مالِك,
• Sho'abah ibn al-Hajjaj شُعْبة بن الحجاج,
• 'Abdurrahman ibn Mahdy عبد الرحمان بن مهدي,
• Abu Ishaaq Ibraheem al-Juzajani as-Sa'adi ^{who could be harder with Kufi narrators} أبو إسحاق إبراهيم الجُوزجاني السعدي ^{قد يتشدد مع الكوفيين},
• Abu al-Hassan ibn al-Qattan al-Fassi ^{who could be harder in his qualification for unknown narrators} أبو الحسن بن القطان الفاسي ^{قد يتشدد مع المجاهيل},
• 'Othman ibn abi Shaybah عثمان بن أبي شيبة,
• Ibn Hazm ابن حزم الأندلسي.

Moderate, but a little lenient المعتدلون مع بعض التساهل:

• Muhammad a-Dhuhaly محمد الذهلي,
• imam Ahmad ibn Hanbal أحمد بن حنبل,
• imam Abu Dawood أبو داود,
• Abu al-Hassan a-Daraqutni أبو الحسن الدّارَقَطْني,
• Ibn 'Ady ابن عدي,
• Ibn Sa'ad ابن سعد,
• Dohaym ^{a hadith scholar from a-Shaam} دحيم,
• Ibn Yunus al-Masri ^{(Father of the known mathematician and astronomer)} ابن يونس المصري,
• Ibn Numayr al-Kufi ابن نمير الكوفي,
• Al-Khateeb al-Baghdadi الخطيب البغدادي,
• Ibn 'Abd al-Barr ابن عبد البر,
• Ibn Manda ابن منده,
• imam Sufyan a-Thawri سُفيان بن سعيد الثَّوري,
• Baqy ibn Makhlad al-Andalusi بقي بن مخلد الأندلسي,
• imam a-Dhahabi الذهبي,
• imam Ibn Hajar ابن حجر.

Lenient المتساهلون:

• imam Ibn Khozaymah ^{(author of the "Sahih"} ابن خُزَيْمة ,
• Ibn Hebban ^{in his Books a-Theqaat and his Sahih} ابن حِبَّان ^{(بذكره بكتاب "الثقات" أو بصحيحه)},
• Al-Hakim ^{(from Nishapur, author of al-Mustadrak)} الحاكم,
• imam Al-Bayhaqi ^{(The known shafi'i scholar)} البَيْهُقي,
• Al-'Ijly العِجلي,
• imam Abu 'Isa a-Thirmidhi أبو عيسى التِّرمِذِي,
• Ibn as-Sakan ابن السكن,
• Abu Bakr al-Bazzar أبو بكر البزّار,
• imam Ibn Jarir at-Tabari ابن جرير الطبري,
• abu Ja'afar Ahamd ibn Saleh al-Masri أبو جعفر أحمد بن صالح المصري,
• Ya'aqub ibn Suyan al-Fasawi ^{(Among his students you may find abu Dawud, an-Nasa-i and ibn Khozaymah)} يعقوب بن سُفْيَان الفسوي ,
  Abu Hafs ibn Shaheen ‏أبو حفص بن شاهين.

The scholarly views on narrators who have only a few narrations

Narrators who have only a few narrations usually are referred to as unknown (Arabic: Majhul المجهول/plural: Majaheel المجاهيل) or having a few ahadith (qalil al-Hadith قليل الحذيث) depending on the hadith amount they have transmitted. They are rather unknown as "muhadditheen": people who transmit what they memorize of hadith. I think I should try to explain what

قليل الحديث

actually means: This term is difficult to quantify and it is somewhat related to ahaad, but still different from it. As for example you may find a whole lot of sahabah who have only transmitted one single hadith or two ahadith. Nevertheless this is the case with the majority of sahabah.
So for this term the era in which a person lives play a role for a sahabi narrating one hadith might count as qalil al-hadith, for a tabi'y narrating only one or two ahadith also, but for the next generations usually only people who are not (yet) known as muhhadetheen would fall into this category, they memorized a hadith or two and maybe quote it while teaching or explaining something else.

There are mainly four different views and categories according my the scholar ibn Amin (same link as before):

1. Scholars who considered unknown narrators as trustworthy by default, even if they don't know anything about them. Among these are:
   Ibn Hebban,
   Al-'Ijly,
   Ibn Khozaymah and
   Al-Hakim (from Nishapur)
2. Scholars who accept the narrations of qalil al-Hadith (even if he had one single narration) and consider these narrators as trustworthy by default: this is the view held by
   ibn Sa'ad,
   ibn Ma'yn,
   an-Nasa-i,
   al-Bazzar,
   at-Tabari and
   a-Daraquotni.
3. the majority view: analyzing the narrations of these narrators and judging their narrations (plural!), if they were on the whole alright they accept them otherwise not: Among these is the majority of the scholars of Jarh and Ta'adil like
   ibn abi Hatem ar-Razi,
   al-Bukhari and
   ibn al-Madini.
4. The hardliners who may even declare a trustworthy or good narrator as unknown. Which are lead by ibn Hazm and ibn al-Qattan al-Fassi.

Some other articles on the topic:

• On QalamEdu
• A Thread
• In a blog