Howdy!
Generally speaking I'd say the Proton and Neutron already have power, and with the cost of power and rails it's more economical to keep them separate. One thing you could do is to get rack ears for the Proton and Neutron and get something like the Tip Too Happy Endings kit (or 2) so all of it could fit in a 9u (or 12u) standard 19" rack.
As for specific modules, Clouds is a classic and Data Bender should work splendidly for industrial effects. Chaos or another Marbles clone from a smaller builder gives a lot of modulation possibilities. Piston Honda is also great for industrial sounds, but do you have a plan for mixing it with the semi modular outputs? I would recommend a quad cascading VCA (like a Veils clone) for that. Personal preference, but I would also recommend an extra filter to tame it : )
As for feedback patching, a matrix mixer is a no-brainer. You could also use the matrix mixer to mix smooth random from Chaos with LFOs or envelopes from the semi modulars to create multiple complex mod sources. Maybe the Doepfer or AI Synthesis matrix for audio and something like the Tip Top Miso for CV (although any of these can handle audio or CV)
Overall, modular sound sources and effects are fun, but really shine when you have the utility modules to support them. Mixers, attenuators, vcas and the like are your best friends.
Cheers!