If you are not ito the idea of feeding adult "standard" rats; you don't have to. I feed even my adult BPs on medium-sized rats alone. They do not get overfat and don't strain to eat their prey. IMO, a large adult rat is too big for most adult BPs, unless you have a really large one.
I just re-read your post though, and it seems you are interested in ASF rats. Sorry; no experience with those! But there are people here who are into them and will inform you I am sure.