pdfLaTeX Computer Modern Cyrillic font in XeLaTex?

Well, not a best solution because there are lot of work around, but maybe someone will find it useful:

(1) In folder texmfsinstallfontstype1publiccm-super there are .pfb fonts that are used when you compile with pdfLaTeX.

(2) Convert them in .otf format using FortForge or some other application. (For the best results copy and extract .afm files from texmfsinstallfontsafmpubliccm-super into same folder where the .pfb files are). For this example I used only upright sfrm*.pfb and bold sfbx*.pfb fonts. Now I have fonts SFRM0500.otf, SFRM0600.otf... etc.

(3) If generated .otf fonts are in the same folder where your .tex file is, use the next code:

documentclass[oneside,final,14pt]{extreport}

usepackage{fontspec}
usepackage{polyglossia}
  setdefaultlanguage{russian}
  setotherlanguage{english}
  
setmainfont[%
Extension = {.otf},
BoldFont = {SFBX1000},
UprightFont = {SFRM1000},
    UprightFeatures={
        SizeFeatures={ 
            {Size={-6.0},Font=SFRM0500},
            {Size={6.0-7.0},Font=SFRM0600},
            {Size={7.0-8.0},Font=SFRM0700},
            {Size={8.0-9.0},Font=SFRM0800},
            {Size={9.0-10.0},Font=SFRM0900},
            {Size={10.0-10.95},Font=SFRM1000},
            {Size={10.95-12.0},Font=SFRM1095},
            {Size={12.0-14.4},Font=SFRM1200},
            {Size={14.4-17.28},Font=SFRM1440},
            {Size={17.28-20.74},Font=SFRM1728},
            {Size={20.74-24.88},Font=SFRM2074},
            {Size={24.88-29.86},Font=SFRM2488},
            {Size={29.86-35.38},Font=SFRM2986},
            {Size={35.83-},Font=SFRM3583},
        },
    },
BoldFeatures={
        SizeFeatures={ 
            {Size={-6.0},Font=SFBX0500},
            {Size={6.0-7.0},Font=SFBX0600},
            {Size={7.0-8.0},Font=SFBX0700},
            {Size={8.0-9.0},Font=SFBX0800},
            {Size={9.0-10.0},Font=SFBX0900},
            {Size={10.0-10.95},Font=SFBX1000},
            {Size={10.95-12.0},Font=SFBX1095},
            {Size={12.0-14.4},Font=SFBX1200},
            {Size={14.4-17.28},Font=SFBX1440},
            {Size={17.28-20.74},Font=SFBX1728},
            {Size={20.74-24.88},Font=SFBX2074},
            {Size={24.88-29.86},Font=SFBX2488},
            {Size={29.86-35.38},Font=SFBX2986},
            {Size={35.83-},Font=SFBX3583},
        },
    }
]
{SFRM1000}

begin{document}

chapter{Глава}
section{Секция}
Текст.

end{document}

Of course, one will have to create all fonts (italic, smallcaps,...) for the best solution. I've used just upright and bold to show how this can be done. Result:

(4) However, I would not recommend this solution, because it use a lot of resources, plus the generated .otf fonts on this way will miss a lot of features; probably there is an easier way. Maybe someday someone create a font package similar to the Latin Modern that will have a Cyrillic letters in different optical sizes.