If Crixus stayed with Spartacus..

...Up to his fight against Crassus, would they have won?

I mean Crixus' army as well.

No. Crixus took out a load of men Spartacus didnt have to. But ultimately they were ridiculously outnumbered.

Rome would still win.

Well it looked like Spartacus nearly won, there just too many numbers, but Spartacus had way more tactics.

Crixus' army was HUGE, and with it he won every battle he was in except the last one.

It's hard to say whether they would have defeated all the Roman armies in the end, but I do feel that they would have done a bit better if they had stuck together. The Romans would still have had a numerical advantage, but I feel like Spartacus utilised his numbers better than Crixus did, in terms of tactics and strategy, so the number discrepancy would have made less of an impact.

Not to mention, the main heroes of the story were total powerhouses, Spartacus took down 4 dudes with a single attack at one point if I remember correctly.

Yeah, the main gladiators are each worth a group of enemy soldiers. Like when Gannicus took out Ilithyia's entire guard escort solo that one time.