The choke spreads fairly quickly and is use for upland hunting such as: quail, rabbits and close range game.
Improved cylinder will be fine.
You can use steel shot with a choke anywhere from cylinder to modified, i wouldn't recommend using steel with a improved modified or a full choke.
if your are shooting buck shot i think either should work fine however if you are shooting slugs choke tubes are not recommended and you shout get a slug barrel which does no accept choke tubes and has no threads
You can shoot slugs in a double. Slugs are best shot through a cylinder bore or improved cylinder choke. The tighter the choke, the more they will be disrupted. There is risk of excessive wear and damage through a tight choke--tighter than Modified Choke.
Cylinder to Full depending on what discipline you are into.
improved cylinder
Basically, no choke at all
Cylinder choke
Cylinder bore- no choke at all.
A Cylinder bore has no choke at all.
Improved cylinder or larger. Actually alot of folks do very well with a skeet gun.
The choke you use depends on your target rather than the type of shotgun. For example: if you are shooting skeet, you would use improved or modified cylinder, but if you are shooting trap, you would use full choke.