To address the pointed questions you asked, go with the 18" barrle for the shotgun. Depending on how it patterns at say 20yds, you may want to use 4s. If it patterns great try 8s ( most of the shot charge in a 12" circle). Rifle wize, I'd say you are fine. Pistol wize, once again fine.
As for mag holders there are many out there, check out your local surplus store for something untill you get to know what you want.
Absolute MUST HAVE are Choate ( brand name ) speed clips for shotgun shells. These hold 6 shells horizontally to the belt in a vertical stack and have a clip at the top to keep shells from falling out. Minimum 2, 3 would be better! As for speed loaders as they are known, these are for open shotguns and requier a special bracket be mounted to the shotgun. MANY club have BANNED the use of these as some have been known to detonate shells when loading ( never seen it myself, but do know one guy that had to have a lot of work done on his arm when one went off.) Practice grabbing 2-3 shotgun shell out of the loaders and then loading all into the shotgun at one time. Your shotgun is the only weak link in your gear due to capacity, but as RickB pointed out, you usally don't get to start a full shotgun anyway. I liked CORREIA's post, use what you have and then decide. Remember you can always ask the guys at the match that seem to do well, In general 3 gunners are good people. Get the Choate loaders however. KURT