I'm heavily considering player Rebels, but you know. Wanna' throw Imperial gear on them.

Is that tournament legal?

Probably depends on the tournament. I used a pair of kitbashed E-Webs with Stormtroopers on them at the RPQ I went to, and no one had an issue with it. As long as they are identifiable as their unit, and it doesn't change their overall height, I don't personally see it as an issue.

Easily identifiable as the indicated unit is the big thing. If you mix the Commando and Pathfinder models together to make Rebel Troopers and include all three units in your force made from the same parts, then it will be an issue. Similarly if you mix Imperial and Rebel kits together, it might cause some confusion as to what the unit is supposed to be.
The final judge of what is allowable conversion wise is the TO of the tournament in question. What is okay at the local level might not be legal at the bigger events.

Look up the FFG tournament rules 😜