It's best to stick with mice. Rats are to fatty and with the slower metabolism if the GTP that can be an issue later on in life when their body it trying to break down their prey item. I've been exclusively feed my collection mice since 1991 and never had any health related feeding issues.
I generally feed my adults once every 2 weeks. It usually takes them than long before they poop and I don't like adding more food until they have had a chance to get rid of the previous meal.
Great looking GTP by the way. Good Luck!