First off.. don't even do what you just said you are going to do. It takes a decent amount of time for fish to get good, fat, and healthy.. and having a yellow tang live in a 10G tanks just so he can eat.. would not be your best option. He will more than likely wind up getting stressed out.. and we all know what that leads to. You should try target feeding the tang. Get a turkey baster or something like that, and have the yellow tang come close to it and then squeeze the food out and visually make sure that he eats the food. You can just shoo the foxface away with your other hand or anything else to keep him away.. it really is not that hard. That's how i feed my pistol shrimp since my clownfish are freakin food hoarders and go for anything.
The butterfly fish is not a relatively hard fish to keep in the aquarium and if acclimated properly and there are no visible signs of stress, longnose butterfly fish are very hardy in the tank. It is just that first introduction to the tank and new tank water that is critical and can make of break this butterfly fish. They are also very peaceful fish, so a community tank will be best and will allow it to live out its life healthy and happy. The tangs and foxface are generally peaceful fish too.. so you should be good to go. Just make sure you follow the acclimation steps 100% correctly and your butterfly fish will do just fine.