To the most extent I agree that you can train any dog not to hurt your ratties.
I had pet rats when I lived in my apartment and they are great! So smart and personable.
As for dogs with your rats, yes, terriers are a bit of a challenge, or any dog with a high prey or play drive will be a risk. These drives are individual in the dog itself, but are probably somewhat influenced by breed.
My SPoo boy had an insanely high play drive as well as prey drive, and it was a trip to watch him suddenly spot one of his stuffies and then move across the room to pounce on and "kill" the evil stuffy.
I wouldn't concentrate so much on "breed" as I would on the drive of the individual puppy or dog. Then you should NEVER leave the two together without supervision.
No matter how much you trust your dog, it is my belief that they are still animals and instinct may override any training when presented with a small, fuzzy, moving target.
BTW, when people would ask me if my dog "bites" I'm the type of person to answer "he's never bitten anyone yet" rather than simply saying "no." This is because of what I said about instinct. A dog is still a dog, and anything can happen in this life.