How will this mod handle that, does it make new nests on top of current spitter/biter nests(assuming the new biters/spitters come out of different nests like vanilla ones) in the world before or does it need to be added to the world at the start.

And as well, will it overwrite nests like ones from 5dim, which I think only change the look of them visually (also possible to know the spawn chances for these aliens/starting evo factor

It removes all units from the vanilla spawners and adds the new units with balanced spawn rates.

If your mod uses the vanilla spawners, it should be loaded after and might work.
If your mod uses it's own spawners, this mod won't affect anything.

The only way to know is to try it.