But could they even contend against the N7 and biotics? I mean Cerberus tried that a lot, and it failed.

More than likely yes, with the combat training a body modification the spartan program goes through. Coupled with their modified cloaking suits and a berserker mentality I think the would have difficulty yes. But they would in the end overcome it. There is also the spartanIIs around as well.

The Alliance has the number advantage and biotics would ragdoll them around.

True enough about numbers. But each spartan completes tactical training rivaling most generals. So I'm thinking guerilla tactics and long range assaults would help to reduce that advantage.