I started with a very small rack and one thing I learned was that a small case was a viable setup as long as it's very focused in its purpose. Especially if you are learning and exploring I would recommend you start with only 2-3 modules you really like, and then figure out what else you need through patching.
Going through the proposed rack left to right:
The ethics of Behringer products aside, an uPlaits might be a better choice due to its smaller size (hp is at a premium here), plus it can load Emilie's final firware, whereas I understand that Brains cannot. The MCO is fine I guess, I know many people like it. You might also wanna consider a Twin Waves for your first oscillator, again smaller than Brains, mutli-algo and you can use the second output as a noise source/lfo.
Forbidden planet is an interesting choice here, but I think a dual/stereo filter or one with multiple outs will do more for you here in the same real estate (frex, the ADDAC dual filter, Dual Dagger, Ikarie or one of the dual Ripples clones).
Maths (Abbacus ) is a fine modulator, but when people wax poetic about it it's usually for its snappy, percussive envelopes, not exactly a hallmark of ambient. It can do a lot more of course (at least Maths can, I don't know how faithful of a clone Abbacus is). Anyway, tl;dr: I feel you definitely need more modulation options and utilities like a CV mixer/attenuverter. Like a poster above said, ambient is about texture and that means subtle but complex modulation of timbre, fiters, panning, etc.
Tallin is a great VCA, no notes. Dual FX I'm not that familiar with, but again, since space is at a premium here, a FX Aid or MFX might fit better in here. Another option is to eschew in-rack effects and just add a pedal integrator module, as mentioned above (although I'm personally not a fan of that option).
Scales is good, but what are you feeding it with? Could use one of the Maths outputs I suppose, but again, some sort of extra modulation source, S&H, random source would be most welcome. Perhaps even a small rudimentary sequencer like a Tirana or Ryo Penta so you can add some intent to your music. Mixup is good.
Final question is how are you taking this out to your speakers/computer? Do you already have an outboard mixer that can take eurorack levels? Otherwise I'd recommend an output/euro-to-line module, or those Mixup knobs are going to be pretty much at minimum all the time.
tl;dr: 1. rethink your oscillator and fx choices to save space, add more utilities and modulation (especially to feed scales and modulate parameters on your voices and filter)
2. optionally look into a more versatile filter and (depending on your situation) an output module