There's plenty of ways to shorthand some kind of Stormtrooper rebellion - we don't need to see Finn and company doing years of on the ground infiltration and radicalization. This is a movie, after all.
You'd just need to lay some groundwork in the first act (Finn talks about how a lot of his fellow troopers were just child soldier schmucks trying to survive or something), develop it a little in the second, then put Finn in a situation where he can make a
Captain America-style speech. Bam, cue crowd shot of Stromtroopers taking off their helmets to reveal a whole new bunch of characters to make toys out of.