To be honest I think people put way too much thought into prey size and feeding schedule. Think about how they feed in the wild, they do not focus on a certain size, they take what they can get when they can get it. The important thing for us is to give them enough to keep them healthy and not too much that makes them fat or go off feed. If your going for accelerated growth or added mass for a healthy breeding season, that obviously plays a role as well. But in general, you'll be fine feeding what you have available. I think it is easier to get consistent feeding habits staying on the smaller side and feeding more frequently. You will be absolutely fine feeding the undersized rats you have available 2-3 at a time and then dropping quantity as the rats grow in size. From what I remember reading of your posts, you are an experienced keeper so you should be able to tell when they are ready to eat again so just keep in mind that reading them is more important than being on a set size and schedule. I know for beginners a set size and schedule is a safer bet but I'm giving you this advise specifically because you are not a beginner. Example, if you give a smaller meal and they are ready to eat on day five, there is no reason to wait till day 7 just because you want to be on a 7 day feeding schedule. Same goes for a larger meal, if they are not ready to eat until day 14, that's perfectly fine. Size of their food and how often you feed really does not matter as long as you are meeting their needs. Just read them, feed them as many as they need to have a satisfying meal and you really don't need to worry about the size. Then just adjust each feeding, when they are ready to eat again, adjust quantity depending on what size you have available at that time. All these rules and charts are great for beginners but learning your animals and having experience, you can take a lot better care of your animals by your own judgment rather than listening to people on the internet or reading a chart.

I will point out that I am not a very experienced ball python keeper but I have many years experience keeping other snakes and reptiles and although they all are a little different, ball pythons are not difficult imo. I have many already and have been able to easily convert them all to f/t and eat regularly based on what size rats I have available. I do feel strongly about keeping them on one food source to avoid problems with them becoming picky eaters. Size, quantity and frequency are not all that important though. I feel reading each individual animal and feeding accordingly is much more important.