Can the same enemy in the same battle have more hp than other enemy of the same type?

Do some / every enemy type have a chance to spawn more durable version of itself?

The vast majority of enemies in the game have a set level. However, there are a few that can spawn with different levels or stats based on certain criteria:

• Commander Grand Horn's level can be different depending on which Fort Condor battle it is, your party's level, and how many enemies you defeated in the minigame.

• Mystery Ninja's level will vary based on Cloud's level.

• Chocobos can have several different levels, depending on what area you're in and which encounter you get.

• Sephiroth's HP will vary based on how many characters are at level 99 and whether Knights of the Round was used on Jenova.

But, to my knowledge, none of those will have more than one of that enemy in any given battle.

The only specific example you give is Whole Eaters in the sector 5 slums. According to my favorite guide for FF7 (which includes all the encounter tables), if there are two Whole Eaters in a battle in the Sector 5 slums, one will always spawn in the back row.

I tried it out, and, indeed, whenever there were two, one started in the back row:

I am therefore going to say that it's likely you were simply mistaken about one not being in the back row. Sometimes they look like they're side by side but they aren't.

Another possibility is that you coincidentally did critical hits one on enemy and not the other. Critical hits do double damage, so that could also explain why one enemy went down twice as fast as another.

For completeness, I'll mention that the Barrier status could result in half damage. It's possible to accidentally cast Barrier on one enemy - for example, if you have a Reflect Ring equipped and you cast Barrier on your party to start the battle, then the character with the ring would reflect the Barrier to an enemy. (This would be rather unlikely to happen with this particular enemy, of course.)