I think this is one of Fox's hardest matchups in the game. I'd give it +1 for Rosa, 6:4 if you like that sort of ratio thing.
I see that offstage is being discussed, which is good. It's pretty hard for Fox to come back against RosaLuma edgeguarding him, he has to rely on fairly linear recoveries that can be interrupted with good timing. Your hitboxes are also pretty bulky, they interrupt both his recoveries. If he hangs on the ledge, you can interrupt anything he does by just holding rapid jabs or timing D-tilt well.
Also, Luma's Jab hit will KO Fox at something stupidly high like 180%. D-tilt is better since it guarantees an earlier KO.
If you watch Larry, you'll notice he'll focus his assault on separating them and beating up Luma. Fox can use D-air and F-throw's hitbox (if Luma is closer to you) to do this. You definitely want to keep Luma close to you more often than not; she makes it much harder for Fox to touch Rosa, without her Fox has a considerably easier time baiting something out of you and punishing.
You should also be careful of Fox's U-air. It can KO Rosa pretty early. Fox doesn't have any guaranteed setups into it nor into his B-air or U-smash, but he has 50/50 setups into them involving dash attack, soft n-air and D-air's last hit. His only throw combos are at low percents, and they should never KO you unless you're in a dumb position (like a platform in T&C going offstage or the top platform of Battlefield+bad DI).
Stages is really dependent on you, I think you're still at an advantage in all of them.