First small rack? Don't you mean first small BIZZARO rack? MUA-HAH-HAH-HAAAAA!
OK, let's make it bizzaro...
Keep the MATHS where it is. I assume you're using a self-contained 84 hp rack, such as a Doepfer A-100LC3; this will have internal power with a built-in DC 'brick' input. If that's the case, this build should work.
Next, add the Batumi and the Poti expander. You will likely want the extra flexibility. So...ok, this gives us the Batumi for modulation, and the MATHS can do a lot of the envelop-ish and slewing behavior.
Then...BIZZARO oscillator! The Harvestman's Hertz Donut mk II should be perfect to compliment the analog oscillators in the two 0-Coasts. This is a very strange but extremely capable dual digital oscillator with internal modulation paths and TZFM capability. The spacing on this also takes care of the odd offset from the Poti, bringing us back into even hp turf. So, what better to go with a BIZZARO oscillator than...yeah, you guessed it...
BIZZARO filter! This thing is brilliant for strange timbres and filter modulations: the Limaflo Motomouth (https://www.modulargrid.net/e/other-unknown-limaflo-motomouth). It's a multiband formant filter based not on typical cutoff and resonance models, but emulating vocal vowel formants and morphing. Put together with the Hertz Donut, you then get into this very strange, alien voice-but-not-voice territory.
Next, not BIZZARO...just a dual VCA from Doepfer, their A-132-3. These VCAs can be switched between linear and exponential behavior, so while one can handle audio dynamics (f'rinstance), the other can deal with controlling a modulation signal's levels being fed...well, most anywhere. Since this is sort of a monosynth (ok, yes, a BIZZARO monosynth), there's really only a single signal path for audio at this point, so having two VCAs like this is pretty optimal.
Next...more BIZZARO! Right next to each other, a Synthrotek VERB, which is a CVable mono reverb, and a Feedback 106 Chorus, which not only modulates the timbre, but spreads the mono VERB signal into stereo in a few different, manually-controllable ways, so that its output goes right to a Malekko Output which gives you a ganged stereo level and steps your signal down to regular ol' line level. Not BIZZARO line level, mind you...just the usual.
Fills all 84 hp neatly. And there's way more than ample room for exploration there...plus it's going to sound NOTHING like the 0-Coasts or, really, most any other monosynth, because this is a...yep...BIZZARO monosynth! Also, you can patch the 0-Coasts in and out of this really nicely in oodles of ways. The SQ-1 could, say, sequence only one half of the Hertz Donut, leaving the other oscillator fixed to create lots of Conrad Schnitzler-esque clangers, or...well, you get the BIZZARO idea here, I presume.
(Last damn time I watch that 'Sealab 2021' episode before trying to get something done! And NO, not something BIZZARO done!)
Sounds right...just keep in mind that, musically, you're probably best putting the modular cab together in a way that it doesn't sound at all like the 0-Coasts so that you can get some contrast. It's sort of like orchestration: sure, you have five different string parts to an orchestra, but then you'll want the wind colors to contrast against the monolithic string timbres. At the same time, tho, those strings need to interconnect with the winds...and likewise, tandemming control patching between the 0-Coasts and the modular cab is a great idea.
Yeah, I might've had some texts shoved up my ass during my undergrad conservatory-type training that I thought were sort of pointless (Piston's 'Harmony', f'rinstance), but Rimsky-Korsakov's text on orchestration wasn't one of those. I've found ideas from that orchestration textbook useful in multitrack and live mixing, studio equipping, arranging timbral structures in electronic media, and so on. Just proves that old adage about everything old being new again...(https://www.amazon.com/Principles-Orchestration-Nikolay-Rimsky-Korsakov/dp/1492248061 )
Oh, yeah...it definitely needs some sort of stereo output module. Even just putting the Clouds (or something like it) last in the signal chain screams 'stereo output'. Get something with a ganged stereo level pot, though; I've been a bit enthralled lately with this one Olitronik output...1/4" balanced outs, metering, headphone preamp, and only 6 hp. Not spendy, either, and the ganged stereo control lets you easily maintain the spatialization coming out of the system. Happy Nerding also has a nifty and inexpensive output, this time with transformer-balanced outs, and having some nice iron in your audio path is always a help, as transformers tend to emphasize even harmonics, making signals processed that way a bit more 'euphonious'.
Oh, hell, yes....those morphing VCOs they've concocted are wonderful if you've got the space. I've heard a number of recordings of experiments and demos with that and its bigger sibling, the E370, and everything I've heard has been really nice. Subtle to extreme seems possible, too, so there's a lot of latitude to play with in terms of coming up with suitable sounds.