You need a mix of front and back.
Depending on the team that you are playing against, you will want 2 backs and a front, or 2 fronts and a back.
If the opposing team plays front alot, and attacks alot, then go ahead and put 2 guys on back, so that they can sweetspot the guys running. Smaller players are harder targets to hit.
For practices, you should all practice snapshooting, running to bunkers, and bunkering.
Also practice loading your hopper with pods in the middle of a game. If you can't load up your hopper with a pod fast, then you stand the chance of getting bunkered, or losing valuable time. Practice anywhere, just taking out the pods and dumping them into your hopper.
For your first tournie, do not set your goals too high. Don't expect to own everyone there. At most local tournies, there are really good teams, good teams, and bad teams. If it's your first time, have fun, and be happy with whatever place you get. Count this as a learning experience.
Whoever you put on back, make sure they can fire alot, and they carry a lot of paint, because they will need it to keep the opposing team down, which is what back players are supposed to do.
Put someone small and fast in front. That way, then can squeeze into positions and places bigger guys can't get into.