For minimal damage but still good stopping power i would say either a 5 or 6 shot 12ga.
Although if your not going to eat the animal a 3 shot is the best
It may vary depending on the game, #6 for squirrels and quail and rabbit, 7 1/2 or 8s for doves.
It will depend on what you are hunting and which brand of shell your shotgun patterns best.
Slugs! If its rifled it is a deer hunting barrel.
Extra full
A shotgun is the only legal firearm for duck hunting in the US.
depends on the use. Hunting? paper or home protection? Federal Winchester or Remington make ammo for this. There are surplus ammo you can find at a lower price them commercial hunting or match ammo. Make sure the surplus ammo does not have corrosive primers.
modified
The one that the man is carrying!
Best would depend on intended use. Federal makes good 30-06 ammo. Soft point for hunting, FMJ for target use.
remington 870 12 gague
That will depend on the individual. Some will prefer a shotgun and others a rifle.
Read your manual, or request a free catalog from Marlin. Factory load is always what they recommend.