If you're intending to change things such that you have captured prisoners under 'tools', I'd suggest that you separate them into prisoners and slaves.
Prisoners would be the hard-won captives from successful raids, and are 'kept in cages' to prevent their escape (this explains why they're items in a stockpile instead of active creatures). They're the ones you use for 'dungeon' reactions, which will presumably work a lot smoother when working on 'component items' instead of actual entities.
Slaves, on the other hand, are captives who've been affected by some mind-controlling ritual, rendering them permanently subservient (and too stupid to do real work). They're the 'pet' type captives, which you might 'train' for battle via the animal training system to add some cheap cannon fodder to your armies, or just keep around for breeding/butchering purposes. It shouldn't be terribly difficult to turn prisoners into slaves, but the opposite reaction shouldn't be allowed.
Aside from that, caste-based skill availability seems a little off. Ghouls can learn ambusher, but don't have access to any ranged weapons, so they can't really hunt... it seems that this should go either one way or the other, and it seems more thematic to have ghoul hunters than skeleton ones. Additionally, given the time-sensitive nature of tanning, the obvious relation to butchering, and the importance of the task to warlocks in particular, it seems like ghouls should be able to learn it.
Similarly, it seems like warlocks should be able to do a few other tasks. Architecture, for instance, is by no means an important skill, but it's required for many special warlock buildings, and it seems more fitting to have a warlock in change of the construction planning. Archaeology also makes sense as a warlock skill, for what good it does. To go a step further, brewing is thematically witchly, and doesn't seem to involve any more 'manual labour' than existing warlock 'chemistry', so that might also be a reasonable warlock skill (especially as regards 'potion' type reactions).