Rainmaker is a bit of a pig when it comes to HP space. But when creating ambient patches, it's a good way to go. I would also look into Tip Top's Z-DSP and the Valhalla series of reverbs.
To save some space you might want to look into the micro or "u" versions of Clouds and Rings. They are 8HP if I recall correctly. They will save you some space for other modules.
I'm anxiously awaiting the Xaoc Zadar to come out. It's four advanced envelope generators in 10HP; perfect for ambiences.
You could also save some space by replacing the Intellijel Metropolis with a 1010 Music Toolbox. I would definitely compare the two and see which fits a little more. The selling point on the Toolbox for me (just ordered it) was the ability to record and loop two simultaneous control voltages from external devices and SAVE it permanently to an SD card. It also has built in LFO generators for all your wiggling delights.
I think you might want a much stronger mixer for blending ambiances as well. That'll chew up more HP than what you have... but might be worth it. Roland makes a six channel mixer with pans and mutes that's in my "must buy" list. It's a lot bigger. But being able to mix/pan up to six sound sources as well as mute them might be pretty handy for ambient mixes.
Oh, and a quantizer... you need one of those. Erica makes one that's about 3HP or you can buy an Ornaments and Crime that's 8HP and get two quantizers and a ton of other features included.
The biggest thing missing would be a clock divider as well as some buffered mults. You'll want a clock divider so you can sync tempos between your sequencer, the Rain Maker, etc. A Pamela's New Workout or a Temps Utile would work wonders there. You may also want to consider a precision adder, attenuverters, and possibly some switches/mutes.
My comments are all over the place... but bottom line, I would research a bit more to find alternates that are smaller and make sure I have plenty of modulation sources.