Are Skiff ever made that do not possess an 'AI', like just a shell without an intelligence integrated? Also is there a variation on how advanced their intelligences get? As in some being rudimentary while others are more self learning?
Is it possible for an outside AI to take control of a 'empty' skiff body?
Canonically, the skiff adopts i've made are only "frames" to put AI cores into. It's like swapping a brain into another body except it actually works and is less horrifying.
Skiff AI core behavior is pretty much indistinguishable from sapient creatures, basing more on "Artificial Instinct" than "Artificial Intelligence" to produce thought. There's a lot of debate on whether this means they possess a soul, but the consensus on Sterilis seems to be that they can't really argue about that until they answer the same question with their normal human meat-brains.
Externally controlling a skiff body is absolutely possible; you'd probably have to just plug in a strong wireless modem where the AI core usually goes.