Yes I am aware of the different amperage on the +12 and -12, I just mentionned the +12 here because as you said it has the biggest load. For now I have 377mA on the -12V so there's still room too. I'll keep an eye on both for sure, and even on the +5V part!

For the mixer, I was looking at the Black Stereo Mixer from Erica Synths. Less features than the Mixology, but still seems to do the job at mixing stereo and the 4th mono input can provide some panning action.

The Doepfer A-119 looks great, thanks for the tip. I was looking at the Entry Point from Pulplogic but it's more than twice the price.