Fruit trees like apple are generally pruned so that they are open at the top, and the branches slopes out and down, so keeping them at 6’ won’t be a problem, you just need to learn how to prune it properly. The more you prune it the more fruit it will produce. There are many YouTube videos on pruning apple trees. They are pruned differently from peach trees.Dwarf apple tree is a great idea! It is easy for me to manage and can be planted in a chicken run.