Rats are fine as long as your balls keep eating them. Ball pythons are notorious for switching for no apparent reason at all.....especially pastels. In my experience with balls on mice, they do just fine on 3-5 adult mice per week. I have one female pastel that fed great on rats up to around 800 grams, just as all of my pastels have done. She has not even taken a live rat in 4 years, but gained 1000 grams in one year just from mice alone. Once they hit the magic weight, they seem to stop feeding for about 6 months or so, then they only want live adult mice. This has been my experience with EVERY pastel I've raised, even my pewters.

To test your animals out, you could set up one cage with 1.2 rats, and 2 cages with 1.5 mice. If your animals feed readily on rats, scrap some of the mice. I would not count out that fact that any ball python can give you feeding troubles when you least expect it.
Good luck.