Grr...had a rather extensive reply on this, but MG logged me out while I was writing it and I lost the whole damned thing. Let's see if I can get the basics down from that again before it bongs me once more.
Drum rack: add some metallic/noise-based sources. The Mutant Machine and Plonk can do these basic functions, sure, but you want to use these for more complicated sound design, otherwise they're a bit of a waste. Look at some little Tiptop stuff for snares, cymbals, hats...the basics...then the other two beefier modules can do some weird variations on those.
The combo of the Field Kit FX and ER-301 is excellent...gets you way off into King Tubby-land or weirder. In fact, probably weirder...which is better!
Consider a dedicated trigger sequencer here to supplement the Squarp outboard one. Plus by using a more 'fixed' pattern sequencer against something more fluid like your outboards, you get lots of potential cross-rhythmic possibilities. I see there's a Grids, but those always stuck me more as a 'sequence manipulator' than just a sequencer, so it would also be a variation device.
Put in some things for making more old-school electronic percussion sounds...a few very basic AD generators + some resonant filters gives you loads of possibilities for sounds such as filter snaps, 'rung' filter sounds like in old beatboxes. Nothing too fancy; you would be just fine with a handful of 2hp stuff here for a space-saving solution.
Add the ability to modulate clocks and triggers...you want something for swing to get things a little more humanized, plus you can shuffle one sequence and not another, then use a trigger combiner (like Xaoc's Bytom) to get flams. Skippers can add a probabilistic dropped hit here and there, making more humanized variation. And then a few clock manipulators plus some logic that you can apply to rhythmic gates can create a lot of cross-rhythmic craziness. Both EMW and Ladik make plenty of space-conscious options that come in pretty cheaply.
Last, look at a mixer that's suitable for percussives, like Hex's Mutant Hot Glue. This thing also has some methods onboard for adding some 'grit' plus compression, and that last bit is important. I don't know how many times over the years I've found myself putting a bus compressor across my drum busses to 'punch' things, make them hit harder and get more in the listeners' faces. Percussion needs to HIT...and compression gets things 'hitty'.
Otherwise, not too shabby...I like the presence of the Elements and Rings for adding plenty odd sounds; it strikes me that the Plonk thru the Rings would have lots of 'abuse potential'.
Will get to the other rack later; don't want to have MG drop me and lose all of this typing a second time...