I have the mod running and the only biters that spawned was acid, changed a few settings and seeing a few common further out now aswell but isnt it suppose to spawn more different types not just 1 per base?

Are the bases limited to 1-2 types or does it increase with evo or distance?

Does Enemy Variation increase the mix? Or does it just randomize stats?

I have almost all factions on except nuclear and poison.

You can find all this info in other threads. Brief answers:
One faction generates only biters of its faction.
Faction is changed only during biters migration (nests expansion via Rampant script). New possible factions are determined by evo. Biters mix a lot during attacks as Rampant makes aliens from different nests combine into one swarm, re-group and so on...
As far as I understood variation makes biters/worms with different health/attack values for the same tier. And yes, it is random. Sometimes it is fun to get a worm with bigger range near your walls than your turrets' range. Advanced fire turrets or rocket turrets from Rampant Arsenal are a must.

I also see poison and nuclear too rarely in 1.* version of mod. Previously they were more common.

Factions don't require migration to change but the effect is slower than intended and should be fixed in the next version