I think you're trying to do too much in too small a case here...
I'd reduce to at most 2 sound sources (and even that's pushing it imo) otherwise there won't be enough room for support modules... although you might be able to use something like the doepfer mini synth voice to a whole voice in less hp...
personally I'd go for something like a mantis instead of this - still very portable - I've carried one many times on planes, trains and buses... and not much bigger - maybe a couple of inches - but you'll fit a lot more in a 3u row than in a 1u row... and then you'll probably be able to cram in 3 voices and still have enough space to adequately support them
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities