Custom monster type mobs spawning when and where they shouldn't be spawning.

Apologies if this topic/issue has already been discussed, but I cannot seem to be able to find the appropriate thread.

Well, the issue pretty much describes itself in the title. My custom mobs that I have set on the "Monster/Mob" spawn setting spawn even when it isn't dark. Even when night does arrive, they spawn before the vanilla monsters do and causing them to oversewn into massive crowds. (Especially in super flat worlds) They also spawn under trees and even in perfectly well, lit-up houses that have plenty of light sources, no matter what spawn weight or setting I try they keep over-spawning even when they usually shouldn't spawn at all.

(Also, in regards to animal and creature type spawning, usually when I create a new world, there's a small group of them that spawns in near me and then I never see any more of that mob in the world again.)

Once again, if this issue has already been covered then I do apologise for not being able to find it.