A lot of older hunters use to load regular B B's, like a red ryder bb gun uses. When the steel shot mandate started, some that I know, took their guns to a gunsmith to have the choke opened , as their guns were mostly bored full choke. The gunsmith told them after measuring the bbl., that the guns choke had been opened. and said if they had reloaded the hulls hulls with steel bb's, that would cause it. So I wouldn't worry too much about shooting steel, unless you use shot larger than bb size. In older sxs, the steel might cause the bbl.'s to seperate at the muzzel. Personally, I use 2 3/4 shells in size 4 and find that over decoys, it works fine. With late season ducks, they seem to be a little harder to kill, so I will use 3 inch 3's, if I can find them. The main difference between 2 3/4 and 3 inch shells is with a slightly longer shell, you can get more pellets in the 3 incher. And it's pellet hits that kills the birds. The faster shot does seem to work better, but the velocity drops off fast with steel, so I wonder if the extra kick on the shoulder is worth it.
If you can, and you may find it difficult, try to see if you can hunt with an experienced hunter. The reason it is hard is that groups develop over time and it's hard to get in with an established hunt group. I did not start in with waterfowl until my early 40's and was most fortunate to find a young hunter whose Dad had taught him from an early age, but the cold was too hard on his Dad's legs for him to hunt. So it was an opposite case of the younger teaching an older.
Good Luck as it is a wonderful sport and the birds deserve your respect.