Oooh...be careful about how you're reading that Mantis power spec! Tiptop's site says it has 3 x 1A on the +12 rail only; the -12 rail is limited to 1100 mA, and if that's exceeded there'll be problems.
Eurorack power uses bipolar DC supplies for the 12V rails. One side is positive, the other negative, and the 'common' acts more like a 'neutral' line for the bipolar supply purposes. While the +12 rail handles the largest loads as a rule, the -12V rail can add up pretty quickly as well, and BOTH amperage criteria have to be watched to prevent overloading each of the 12V busses. My suggestion is to find powered case solutions where you have suitably large amperage potential on all rails to avoid overtaxing any parts of the power supply setup.
As for the output/mixer issue...if you're going to mix down to mono (which would be what the TriATT would allow), just get a mono output module. Stereo mixers would be better, though, because you're going to want to step up to a spatialized output signal eventually, so my suggestion would be to look into a small performance mixer that allows CV control over audio levels (via exponential VCAs in the mixer), panning, an AUX send/return for paralleling a global effect, and mutes to drop parts of a global patch out to immediately vary the sound. Qu-bit's Mixology is probably the best value in these at present...four channels, stereo, AUX send and return, and so forth, all under voltage control for $399 which is actually pretty cheap when you consider what's built into it. And if you already have a Clouds, running the Mixology output into it, then on from the Clouds (using the Clouds as a post-mix 'playable' processor) to a stereo output would be the way I'd go.
Input-wise, I've found that Doepfer's A-119 is probably the best feature-to-value module of that sort.
Also...when speccing modules, try this trick: divide the module's price by the hp width. The lower you can get the resulting number, the cheaper the overall space utilization becomes. One of Eurorack's little secrets is that when you have room to spread out and use larger-width modules, the price-per-hp tends to go DOWN, so staying in tightly-confined cabs can actually be more spendy with respect to space used than going with something where you can use bigger modules.