If you can see a noticeable lump in your RTB after feeding it a rat, it's not ready to eat guinea pigs. I am quite sure with these boas you just feed prey items that have the same girth as the thickest girth of the snake (when not recently fed). Once your boa can eat large rats without any sign of bulge, you can try guinea pigs that are a little bigger in girth than rats (or small rabbits).
I personally don't own any RTB's so haven't fed them guinea pigs LOL. I do have a Brazilian Rainbow Boa who is currently eating small rats. BRB's are slender boas so will never possess the girth that red tails possess so I doubt I will ever feed her anything larger than a typical full grown rat.
Personally I think it would better to get rabbits instead of guinea pigs..I beleive rabbits are cheaper to buy than guinea pigs (but I could be wrong, never actually priced feeder guinea pigs). Even breeding guinea pigs can be problematic. For one, they have small litters (2-3 being average), gestation is long (I beleive it is something like 45 days) and females often develop problems delivering, especially if bred too young or are overweight (or have had several litters already). They also need alot of room compared to rabbits. Rabbits do produce much larger litters, but cannot become pregnant while nursing a litter, so that does slow things down a bit, but staggering the does who are pregnant will keep a steady supply. That is about all I know about rabbits LOL. One advantage with guinea pigs, is the babies are born fully furred, eyes open and a pretty good size (about the same size as a weaned rat) and mature fairly quickly from there (2-3 weeks and they are weaned and can be separated from the mother..and they should be separated by sex too).