You are allowed to load 8+1 in Limited shotgun (IIRC). A 20" barrel and an 8-round tube are approx. the same length. I use cylinder choke, but I've shot a couple of matches where cylinder and birdshot were not enough to knock-down steel that could be engaged only from a single position - that is, you couldn't get closer to the steel to knock it down, you had to take the misses. I know a guy who shot everything - bird, buck, slugs - through a full choke, and it didn't seem to harm anything.
How your rifle is set up depends somewhat on what kind of stages you'll see. The last couple of matches I shot featured 300+ yard shots, and for that, I like the extra sight radius and velocity of a 20" barrel. Another match I shot featured no shots longer than 200yds, and a house-clearing stage, and on that one, a carbine was the way to go (it was fairly comical, watching the Open shooters maneuver their 22" barreled, scoped, bipodded guns through rooms not much bigger than closets, to engage targets at 5-15 feet). If there's lots of long-range shooting, a fast twist (1:9-1:7) and heavier bullets would probably be the way to go. I shoot 55gr ball, as it's fairly cheap, and doesn't punish the steel targets.