Without a solid repeatable cheek weld, you won't hit anything, and it doesn't matter if you're shooting a .22 Short or a .600 Nitro Express. Suffice to say, if you haven't mastered the principles of basic marksmanship, and developed suitable gun handling skills, a real world defensive shooting scenario is not the place to learn. It is advisable to use low recoil ammunition, such as the law enforcement, or managed recoil stuff, while developing these skills, since it allows longer range sessions before fatigue forces you to stop, or worse, you develop a flinch. But a gun that fits correctly, combined with a good shooting technique are the best solutions to recoil management.
If the LOP is on the short side, the effects of felt recoil are less than if its too long. At the beginning, confine your shooting to off hand, that is, take a boxer's stance with your feet placed approximately shoulder width apart. With your forward foot towards the target, and the gun at your shoulder, flex your forward knee, and shift your weight over it. Once the gunbutt is positioned in the pocket of your shoulder, lower your elbow to close the pocket and protect the shoulder joint from recoil. Position your cheek in contact ith the comb of the stock, in such a way that you can see the sights or the bead over the height of the receiver, put the front sight on the target, shift your focus to the front sight, and press the trigger. The result is a hit.
Once you have the basics down, and have come to the understanding that 12 gauge recoil is inconsequential, you can begin shooting from, kneeling, kneeling supported, sitting supported, prone, and roll-over prone positions. By this time you should be able to start adding full power loads to your repertoire. Confine your shooting to typical defensive ranges, say from 3 to 25 yards. From this point you can begin work on snap shooting techniques, and moving targets.
On the question of home cast shotgun slugs, these are unlikely to prove to be as effective on game as the heavy Brenneke style slugs. Shotgun slugs are not made like rifle bullets, rather they are essentially thimble shaped projectiles, and under near range, full velocity impacts, the thimble has a tendency to collapse, and the resulting pancake, produces little penetration, the result being a wounded animal. What makes the Brennekes and their various clones different is the pedestal that the wad attaches to. The pedestal provides greater support for the integrity of the slug, yet allows the slug to compress, and pass safely through a choked barrel.
