So jsut lets have a look at Andromeda - a new galaxy to be colonized by a multi-species endeavour. An unknwon, uncharted galaxy, with countless possibilities for trouble. What do we need to fill roles to make it a multi-species success-story?
Krogan - muscles. We meet a hostile species, the ARK gets attacked, we need to rid a planet from evil critters? Send in the Krogans, the dead cost nothing
Turians - the brain to the muscles. The scalpel to the hammer of the Krogans. Perfect planners, no other species calibrates as they do. No nonsense.
Asari - long lifespan, psi-powers, can easily multiply with many species, probably rather good diplomats whcih is essential to the new lands we visit
Salarians - Great scientists, very creative, problem-solvers.
Quarians - expert technicians, especially in terms of space-flight and all of this. Also great at improvising and accomplishing stuff with minimal ressources
Humans - coolest race of the galaxy, destined to rule them all, future masters of the universe. Also probably more adaptable than most races it seems (beyond physical)
All the other minor races? Either hostile (Batarians, pre-ME3-Geth/parts of 'em at least, Vorcha are barbarians) or simply underdeveloped in terms of skills and background (Elcor, Hanar, Drell), except for the Volus, actually. I could see a place for them in such a new planned society - they are kinda good merchants, or not? Trade and negotioans will be an extremely important factor for such a longterm-venture. As long as you keep Bionics out of their hands - we do not need godlike