First of all, it's too late to prune now. Let that branch grow and evaluate it next spring.

So prune when dormant to what height? Should I stake it to keep it growing straight? By the way, I cut off the parent at the trunk about 3 inches off the ground before this baby took off. I just didn't have the ability to pull the stump at first. Also, is it common for fringe trees to sucker like crazy? It has always done this although I bought it as a "tree".