Diablo – What are the requirements to unlock the seasonal transmorg reward

Yes, but there are some significant caveats.

1. Many pieces of gear affect how much experience you get from monsters. When comparing with others, you should remove all such gear.
2. There is still some debate about grouping with higher level heroes. I'll have more updates on that, in that question.
3. A monsters level depends on the difficulty level.
4. Remember that when your level changes (typically up), monster experience scales down.

There is a very helpful table here which lists the experience scaling for your level compared to a given monsters level. It is also untrue that you can't determine the level of a monster - but I don't think there is a way to determine it in game. I've also been taking measurements for other questions in various parties, and I've found the output to be consistent with the conclusion that monsters of the same level always award the same amount of experience.

If you manage to rescue 20 people, you will receive the Hero of Caldeum achievement.