BPs are good choices for a first time snake owner. As far as yearling versus baby, for you I would recommend the yearling. It has already had a good start and has taken several meals. I wouldn't want you to be stuck with a baby that may take more patience and time, as I would want your first experience with snakes to be a good one, and not frustrating and discouraging. There are always exceptions, but for the most part BP's tame fairly easily. If you want to handle your snake a bit, start slow and easy. If he seems tame enough, but stops eating, it could be a sign of stress, so you would want to hold off till he's eating again. That 55g tank is a bit big as well. You may want to partition a section off and provide lots of hides until he grows larger. Keeping the humisity up in a glass tank can be tricky too, but not impossible. I keep all mine in glass tanks, and there are a few links on here that people have posted about keeping the humidity right in glass tanks. Anyways, good luck, and post pics when you can, as we simply cannot get enough around here ! Oh, and welcome to the forum! glad to have ya!
PS-what kinda tortoise/turtles do you keep? my boyfriend has a Russian tortoise.