I've at 40% evolution and facing tier 5 rampant enemies as intended but most of the enemies are still the vanilla small/medium biters with their vanilla 15/75hp. I've noticed the vanilla nests don't use the health multiplier from the settings either. Is this intended that they don't scale with the rampant enemies stats?

Yes. Standard biters will remain as they were made by the authors.

Is it possible to add a setting to override the vanilla biter/nest stats to follow rampants?

No.
Changing the parameters of vanilla biters should be a separate mod.