Inboard flaps essentially increase the AOA near the root compared to the AOA near the tip. That means they act like 2 tons of washout.

BUT, if you don't really have flaps, and you use your outboard ailerons as flaperons , then you aren't getting the washout effect. You are getting the opposite. So the fist thing to stall will be the tips, not the root. I wouldn't do it.

IF you need flaps , then add them later. But, (and it might just be me), I have not flown a Mustang that didn't actually land better without the flaps.