For most wingshooting I'd use 26" as a minimum. Any shorter than that and the gun gets "whippy". Yes, it's possible to hit a target on the wing with a 24" barrel, but it's harder.
That depends on the indian. My 20" 20 gauge coach gun is DEADLY on doves out to 40 yards. I choke it I-C/Mod. Choke is important for range, barrel length is superfluous. I'll admit the coach gun is better at close birds dodging and darting than those high sweepers, but I hit my fair share of high sweepers. I was getting a lot of high sweepers last year, birds moving pretty good at 35 to 40 yards, and shot about 50 percent which ain't bad on doves, not for ME, anyway. I don't do any better with my 28" Winchester 12 gauge. This season's trip to Waco to open the north zone, I used the 20 in preference even though I had my 12 along with its 28" barrel. We were hunting tanks and the birds were in close and weaving around looking to land. It's been so dry here, the tanks were the place to be if they had water in 'em. We massacred 'em
My buddy uses a light O/U with 26' barrels and did well, too, but I outshot him as I usually do.
What I bought the coach gun for is motorcycle trips to dove hunt, breaks down and stashes away real handily out of sight on the bike. When I had my old Goldwing, I'd just stick it in the top box, could lock it up. On my KLR, I just stick it in a backpack with my ammo and water and hunting seat and strap it to the rack. Can't do that with a 28" repeater.
You have to concentrate on you swing and follow through with the shorter barrel, for sure, but I have proven to myself that I can be just as effective with my coach gun on doves. The novice wing shooter probably should stick to more forward balanced guns, though, as swinging with a longer gun is more natural.