We deal with a snake breeder who does rats as a sideline. He's actually a pretty gifted breeder of rats. He told me he started his line by crossing a strain of large fancy show rat with another strain of small rats. I'm thinking smaller rats may be beneficial for a snake breeder that would typically have smaller juvenile snakes to feed?
But the upshot of this decision seems to be some very healthy rats. Some research suggests that dwarf rats don't commonly get mammary tumors, so tumors may be linked with growth hormones... Our oldest girl Cloud is pretty pudgy and a year and a half old, but she weighs only 1 lb and still fits into a small igloo. Our more common feeder rats grew fast and made it up to over 20 oz. but got mammary tumors as did our huge high white show type rat.
Breeding is, in part, science and in part art, to some degree it's a gift and to some degree it's luck especially if the breeder is wumping up their own strains. I've seen my share of wild rats, and for the most part they are smaller than our domestic rats. So I'm not going to suggest that a smaller strain is a bad thing, in fact it might be good healthwise. About the only way you are going to know if your breeder is any good is to see how your rats do as they grow older.
I asked our breeder about his rats health and he flatly said, "They are healthy and they don't get sick." To be honest, it seemed like a pretty pat reply that the worst kind of BYB might give... But so far of the three rats that we and a friend adopted from him... they have in fact been perfectly healthy and they haven't gotten sick. They have great temperaments and all three go outside with their owners... Our two have qualified as true shoulder rats.
So while you have raised several red flags when it comes to your breeder, that doesn't mean you didn't adopt some very fine rats. Only time will tell. Small is not necessarily bad.
This is Cloud at around a year and a half old...
And here she is with Misty about a year younger... The fence is standard size for comparison.