Whether or not you want to put both fish in a 55 gallon is up to you. Read the link that Craig posted and make your decision. All I can do for now is answer your first question
Tangs (and many other fish who are aggressive towards their own kind) will most likely fight if they share the same body shape. Since the yellow and hippo are not "conspecific", they will most likely get along as long as you add them at the same time. If you can't add them at the same time, you'll want one tang to be larger than the other. I would suggest that the hippo be the larger fish since the yellow tends to be aggressive.