"Don't you ever say that Snipers are cooler than Assaults!"
In my opinion, you want two Assaults when you get Squad Size I. Then when you get Squad Size II, you either take two Snipers, or two Heavies (moar rockets! Also, if one of them takes Holo-Targeting, you can just have him shoot or suppress the toughest enemy first, then have the rest of your group all get a +10% chance of hitting him. Including the Bullet Swam Heavy).