I believe a "Skeet" tube would probably be considered the 'normal', if there is such a thing. I've always used Cylinder and Imp Cylinder with equal success(several 25 straights). Skeet shooting is fairly close-up when compared to other clay sports. No need to over choke when starting out, and it's hard to be under.
Also, I don't consider myself a 'Skeet shooter', just someone that 'shoots skeet' when I feel like a little change of pace. I'm not really qualified to give advice on the sport, but have shot a few dozen rounds in the last couple years.
Barrel-length will always be a personal choice, I think. My brother uses a 24" on Skeet and Sporting Clays because he's more comfortable with that length. I've always used 28 inchers for everything(Sk, SC, & Trap).
Barrel Type? You will find most sport shooters use a ventilated-rib barrel. Skeet and Sporting Clay ribs are usually low to medium height, while many Trap shooters use higher elevated ribs. I've tried skeet with a bead-sighted barrel before and can't hit a damn thing, even though I grew up on bead sighted shotguns. This doesn't include ventilated-ribbed barrels with a bead. I'm talking about bead-alone barrels.
As a little side note-
I was out on DPMS's range last week where I shoot trap league(I'm on the Panther Arms team). From a distance, I watched a shooter doing very well on the skeet range, knocking everything out of the air. One of the workers walked up to me and said, "Not bad for an 87-year-old using a .410, is he?" The shotgun was an over-under with about 26" barrels. Not sure about the make/model.
That last pragraph just shows that even a .410 will do the job. A 12-guage just makes it that much easier. Barrel-length and gauge aren't really as important as stock-fit and swinging capabilities. Those Skeet targets look like they're doing Mach 1 the first few rounds. No matter what choke you use, it seems like you either miss big or hit. A barrel that swings good in YOUR hands will be the greatest aid in breaking clay.
The adjustable choke assembley mentioned above will add a few extra ounces to the end of your barrel. This can be either good weight or bad weight, and will change the gun's balance. Also, the only style I've seen has used multiple pedals of steel to constrict the end. I wonder if these overlaps might cause wad/shot disruption???...though, I have no personal experience with them. I guess I'm just happy with hassle-free, one-piece tubes.
I hope this helped some. It's all I know about the subjuct of Skeet, which isn't much. Good Luck