In my experience .Introducing a fresh bloodline ie bull x to a terrier strain and by that I mean a bull x terrier 1/8,1/4,1/2,3/4 is based on how long the line of terrier has been about to absorb the introduction .
All lines arnt equal .The older the line the easier it is to breed out unwanted additions like size .New creations will need time to breed out selected traits .