It is important to transition slowly. Don't forget that this new dog is in his territory.
In order to get things going on the right foot, is to get outside and go for walks so that they can bound more on neutral ground. If they engage in play outside then it should go well down the road.
Both dogs should not be forced however to interact. They should ALWAYS be supervised until they settle down.
The new dog should be crated when you are not in the home. Very important.
Also, what do they fight about? Is it about a toy? bone? sleeping space?. Is there are problem when the new dog is close to you?
Do you know how to read body language? It is important to find out what the triggers are in order to correct the situation. Correction however does not mean yelling or shoving. It means to difuse the situation before it happens.
Good luck. I think if you are having problems you should consult with a dog trainer to help access the situation and perhaps provide you with some tips. It is one thing for me to type this out, it is another to actually see the situation and provide viable tips.