I have big probs with two of my dogs, fighting, great dogs apart but together is just not happening.?

dog 1 is 8yrs jack russell /springer cross. dog 2 a West Highland terrier 8yrs. They have been together for 4yrs with no issues. I recently took a re-home 6month puppy a jack russell. I was going well and then all hell broke loose. The two jack russell have begun fighting. The older dog is not castrated and I have recently had the 11month pup castrated. I cannot allow these two dog together as they fight. So far no bad bites but I want to avoid this. Presently trying slow introduction but no progress......Any ideas?