Based this walkthrough posted on GameFAQs (search for {39} in the walkthrough), the items you receive are dependent on who you send which direction. The walkthrough author recommends the following orders:
- Cloud: Left/Down
- Barret: Left/Up
- Tifa: Right
- Red XIII: Left/Up
- Cait Sith: Left/Down
- Yuffie: Left/Up
- Vincent: Left/Down
- Cid: Left/Up
You then enter the path at the bottom of the screen. I haven't verified this myself personally, but you can always try it and either don't save or use another save slot until you can confirm this gets you the items you want.
Note: Once you have completed your path and enter the room with the rest of the party members for the first time, do NOT talk to any of them. If you do not talk to anyone and continue through other doors in the room, you can actually go back along their paths, grab loot from all of the chests on that path, and still get loot from the party members when you return to the room. This is important because the above orders are intended to get you an extra Mystile, Imperial Guard, Shield Materia, and Counter Materia if you explore and loot all of the paths before talking to anyone else in that room.
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.)
Best Answer
Have a look at this thread comparing stats of the characters:
image from above forum post
Note that the difference in stats is really pretty minor, and by equipping materia to any character you'll see more variation in stats than you will between characters, if I recall correctly. Additionally the Materia system allows much more versatility than just making someone a "healer", you don't have healers so much as "people with Cure and Revive", so it's easy to slip healing into any team.
Aside from their limit breaks, all characters are basically blank slates when it comes to their role in battle. Note that while cloud is the second best at magic...he's also definitely the best overall and is easily suited to any role. Basically pick a team of three favorite characters based on their personality/looks/limit breaks/weapons/whatever you please, then assign roles as you see fit. Since Cloud is basically a given and too overpowered to dedicate for healing, my healer was always just "Not Cloud" or occasionally "Not Cloud #2", though I usually gave multiple, or all, characters Cure.