Fish are like people, take 5 people you don't know and make them live and work out of a house. Then try to guess if they will get along. Sure there are rules of thumb to follow but other than that your guess is as good as mine. Any time you add fish to your tank there are only a few outcomes. They can get along like flint, they can fight it out and then settle, they can fight to the death then there is no problem after that, they can fight and you can take the new one back out if you can catch it.