If we're going to have four to six people, I'm guessing you only want one of each class? And I just like the Merc idea because having Spectres would basically limit us to three species and the threats we face would have to be unbelievably tough to be credible. Like ME 2's Suicide Mission on Steroids. 

Hmm should I be a Turian Sentinel or a Drell Vanguard? Choices, choices.