Since it's referred to as the "Realm of trapped souls" I think it would be interesting if all of the people and animals you soul-trapped fought you while inside. So on one playthrough you could encounter a few wolves and a mudcrab and on another you would find some bandits and saber cats. It would be limited to a reasonable amount because it would be pretty unreasonable if there was a hundred bandits in one room.

no theres a difference between trapped souls and souls that you have soul-trapped. When you soul trap something you temporarily bind its soul to one of your soul gems so if it dies while it is still bound to your gem then the soul remains trapped in the gem. Soul cairn I think is more accurately described as the realm of lost souls that are now trapped in soul cairn, it has nothing to do with soulgems as far as I'm aware