Glad someone else mentioned it first because I didn't want to speak out of school, but I have also been told that the first 3 years a re the growth years. I've heard that you can (for lack of a better term) power feed during this time and get them up big, or just keep them on a normal regular feeding schedule and get them to their standard 6-7 feet and then switch to maintenance feeding to keep them at their size.
I've also been told that getting boas on guinea pigs or rabbits isn't recommended. I guess rats are and should be good enough for the duration. I don't know if it has to do with the digestive tract being able to handle such large prey items or what. I also know several people who do feed their boas large prey items with no adverse affects. I keep mine on a regular feeding schedule (once every seven days)