Don't do this. You're majorly painting yourself into a corner by using a cab that's FAR too small to accommodate all of the functionality you're aiming for. For one thing, if you have to jam ALL of one function's voicing into a bunch of 2hp modules (which are a pain to adjust in the configuration your actual build has), you're doing something wrong.
Generative ambient...besides not really being very percussion-centric...really requires a lot of control manipulation subsystems that you don't have room for here. Think more along the lines of a machine that has a nonrepeating patch program, and once it's set into motion, it continues to create without performer interaction. THAT is "generative ambient".
I would stop right now if I were you, and go and get a copy of VCV Rack. You need to see firsthand how this should work, and what it really requires, and without the intermediary of VCV Rack, it's going to be far more difficult to pick up on this. Right now, you're building a money hole. You don't want to do that. LEARN the proper functionality requirements, THEN move to hardware...otherwise, this sort of thing gets real spendy real fast.