Use auto or pump shotgun, may want to go with a shorter barrell 24" or 26". For 12 gage, I'd be tempted to use either #4 (lead) or # 4Buck as a general all-round shell. Check out your patterns and penetrations. I prefer full choke, but for if strictly for defense, one of the other chokes would have a slight edge in that they spread the shot out sooner.
# 4, 5, 6's for goose to dove size birds. 7 1/2s for smaller.
Buckshot for gator type and hog size critters. #4B, #1B, #0B, or #00 B.
#4 or # 4Buck (or BB's) for big rats (rabbit size)
(if these are hog-size ones, then #4Buck would be minimum)
BIG Snakes - #4, also #5,6,7 1/2s for smaller ones, #4Buck or other buckshot for Anaconda size.
Keep in mind that for hunting I'd used specialized shells just for that animal. For defense load, either #4 or #4Buck will penetrate most satisfactory within a few yards, but for hunting, say 40 yard shots, these would not have the necesary power for say hog size or gator types animals.
Also if your hunting, a centerfire caliber rifle, say .223 up to 6.5mm would be handy or some of the bigger animals.