We would use a modified or full choke, and #4 shot.
Extra full
Extra full
No
Can you Yes, should you NO. a full choke should not be used to shoot slugs.
No no no
Mossberg has a web site.
no
Many of the Mossberg bolt action shotguns used a removable choke, not an adjustable. Sold under the name C-Lect-Choke, this was an external choke that could be unscrewed with a special wrench, and a different choke screwed into its place.
Mossberg has a perfect combination for your boy friend. The 500 can be purchased in assorted camo colors as well as traditional wood and synthetic stocks. The combo choice offers two different barrells. One is a 28' turkey barrell with a super tight turkey choke tube as well as a slug barrell for deer. They also sell a combo with the afore mentioned barrells as well as a barrell for waterfowl giving you three seasons with the same gun. Changing the barrells takes only minutes and the guns are very well made.
C-lect choke is an adjustable choke system that Mossberg implemented on their shotguns pre-1977. After 1977 they went to the accu-choke system. The choke is adjustable between Full, Modified, and Improved Cylinder. It is, however, not recommended for use with steel shot.
You will need to change barrel's, a accu choke is not on a slug barrel.
Check the owner's manual, or check with Mossberg customer service. Unless the choke is rated by the maker for steel shot, not good to do.