Look through other posts about barrel length and turkey hunting. There's a lot of informative responses given. The main reason turkey barrels are made short is for weight considerations and lugging a turkey gun through heavy cover.

Short barrels for wingshooting....not a good idea. A lot of shooters assume a shorter barrel will help them point faster (snap shoot). Shortening the barrel will move the balance point backwards, making the gun stock heavy. A stock heavy gun will be more difficult to swing smoothly. You may be able to point at the right spot when swinging on a bird, but can you keep it where it needs to be?

Everybody's different, but 20" for upland seems too short, unless it's for a dedicated youth gun. (or you add weight to the fore end to move the balance point forward) For upland shooting, a barrel in the area of 26" ought to do.

It's easier to convert a shotgun for wingshooting to turkey than the other way around.