I think you handled it well.
I would play with them a lot and see where the trigger for the fight is coming from. Get any toys they may fight over and let the dogs know you won't tolerate any aggression from either.
Because you new dog is a rescue he may have had an experience in the past where he needed to use aggression.
Do you keep them separated most of the time or are they together?
How do they get on the rest of the time?
Do they resource guard items or people?
I've seen dogs fight and be fine after. It depends on the situations and how you deal with it and then how you act in future. By being more cool with both dogs can be good for a while rather than being really petty.