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If you are going to spawn every single pixelmon by hand or command blocks then I would use a superflat map , just make sure it looks nice
The biome also doesn't play an important role if you spawn them on your own. So I suggest using whatever biome makes your cages look the best e.g. forest biome for nicer grass instead of swampland or stuff like that. And you should disable spawning or you will get random pixelmon spawn too.
[5:47:56 PM] AnDwHaT5[Pixelmon Staff]: HIDE YO KIDS HIDE YO WIFE
[5:48:05 PM] AnDwHaT5[Pixelmon Staff]: ANIMAS COMMING TO TOWN BISH