Some dogs just do not get along, and your purebred may be the one that is starting the problem. You cannot relax anymore around your dogs. As soon as they are in the room together, you need to down the dogs and give them treats when they obey. This may or may not work, but you have to be consistent, and never let the dogs run around together. After a long time, the dogs will probably forget their aggression and be okay with each other.