I wouldn't carry FMJ for self defense, not because it won't work, but because it doesn't work as well as HP ammo against critters both human and animal. There's enough times when HP ammo doesn't open up for one reason or another, in which case you may be shooting solids, like it or not.
I sometimes carry mixed ammo in my magazines (usually as spares) but they're all HP ammo. The round count in the boxes (20,25,50) just don't divide evenly with most magazine capacities so there is always 5, or 2 or more bullets left over. Eventually they accumulate and get stuffed in one magazine. Since I buy my "premium defense ammo" when I can find it on sale and I'm not overly picky about brand, I'll have mixture of "left overs" from Winchester SXT's, Remington Golden Sabers, CCI Gold Dots, Federal Hydra shocks, etc. They all function fine in my pistols and shoot to the same point of aim so that's not an issue.
I guess if you're really on the fence about which ammo is "best" to your mind then a mix may be one way of to compensate and avoid having to make a decision.