[@AeronVex]

If you want players to be able to choose from multiple races then you can't go with a species based military. The Normandy was the exception, every other human ship would not allow alien crew members same goes for the turians, asari, salarians, etc.

If you want players to be able to play geth like Legion then I suggest a post ME3 timeline where Legion uploaded the Reaper code and destruction was not chosen. Legion was unique and only one like it was ever made, so the only way to play as one is for Legion to make them sentient.