I am sure most will recommend that you do not let the tree put out fruit the first year, even if it wants to. I think that one year after a 15 gallon, is probably ok, 2 years for a 7 gallon. but this really depends on the size, I have seen some plant in a 7 gallon , that really look big for the size container they are in, and then some that look a bit small for the 15 gallon.
In any case, you want the tree to get its roots established, once you transplant,
As for varieties, you know your opening a can of worms in this forum, everyone has their favorite, and some love/hate the Carrie for example, personally I like it. safe to assume you have tried these, if not, I recommend you do specially the Carrie If you tried it and like it, wonderful. Kent, is a good late variety, Keitt is a bit later but I prefer Kent. for mid season. you have tons of varieties to choose from.