From my experience, 12ga and 20ga are most common in shooting sporting clays and upland birds (pheasant, grouse, chukkar, etc.). Although the last I heard, 16 gauge was coming on. I saw almost ZERO people shooting 28 gauge or 10 gauge. 410's were often used by kids and/or females.
When I shot sporting clays, I used a 12 gauge. The shells were 3/4 oz, 7/8 oz, or 1 oz of number 7 shot. Sometimes I'd use 7-1/2 size shot. These were plain lead. I believe they were Remington STS.
Check your local laws for bird hunting. In some states it is now law that you must use steel shot for bird hunting. So if you are going to cater to both bird hunters and sport shooters, you may have to stock both steel and lead.
You may also want to feel out your clientele to see if they are reloading shotgun shells. I reloaded my 12 gauge and was seeing a savings of about 50% over buying off-the-shelf. If you get people wanting to reload, you'll need to stock primers, powder, wads, and shot. There are many varieties out there. You may want to talk to your customers to figure out what to stock for this.