Any shotgun can be used for trap. I once saw a 10-ga black powder lever-action gun used at a local club.
In terms of other shotgun sports, like Skeet, other gauges are "required" 12, 20, 28, and .410 are used for a 4-gun match.
For "Real" Trap, 12 is used, as it throws the most shot, and therefore, gives the shooter the best chance of breaking a bird (A 0.5% difference in bird count (1 bird of 200) or even less, can make the difference in winner and second.)
Amateur Trapshooting Association (ATA) rules specify 12 as the largest gauge, 1 1/8 ounce as the max charge weight, and 7 1/2 as the largest shot. Why would you not use the most effective tool in competition?
That being said, my son started on a 20 with a modified choke, and had a 24/25 on one round....so it can be done. Sicne he turned 11, he's been using a 12.
If you have a 20 that fits (no fit,no hit), you'll do well. Just shoot.
Come on out to Southborough Rod & Gun Club
www.srgclub.com and join us!