So I've been playing this a lot and I've been thinking about good squad combinations, the first I could think of is a tank oriented squad... mostly for attacking purposes:
Two guys in the tank: one driver and one in the turret (they're classes don't really matter)
One engineer: that's all the time with the tank whose duty is to repair the tank and rpg any enemy tanks to give the friendly tank an advantage. also to spot enemies and possibly pick off anyone who tries to c4 the tank
One assault class guy: he would be pretty far from the tank and would usually be the one to arm the crates and spot/kill any threats (turret rocket launcher, enemy engineers, c4 wielding enemies attempting to flank, etc)
as far as an all out infantry class:
sniper: main duty is to spot people and when possible kill
two assault guys: arm/defend crate from a relatively close distance
one medic: revive people provide support with lmg
these classes are most useful when attacking, I don't think the medic is as useful when defending since you have unlimited spawns. It's still convenient but not as useful as having a grenade launcher or an rpg.