Mossberg 500 in 20 gauge is a fine weapon, I gave one to Mother Slim back in the 80’s. Really, there’s no real difference in 12 vs. 20 gauge ballistics, just the weight of the projectile(s). Most 12’s throw 1 1/8 ounce at 1200-1300 feet per second, versus the 7/8 to 1 ounce in a 20. The 20 will be a smaller frame gun and easier to handle.
For a handgun I’d suggest a Smith & Wesson .357 in a K frame or L frame 4” barrel, and shoot it with .38 Special +P rounds. A variety of grips are available so it will fit just about anyone’s hands. The Pachmayr Compact is a good place to start.
The biggest hurdle to actually getting these things will be Mrs. Fierro.
You’ve met her. She’s a real chicken.