How to get a 900g female to 1500g in 6 months...???

[Corrupter]

Ah, Our definitions of "small rat" seem to be different. I consider a small rat to be about 100g give or take.

I think it would be perfectly fine to feed an 80-100g rat once per week.

I agree. I think a medium is too big for a 900g ball but my mediums weigh in around 150g. A small every week is fine and should at least put them over 1200-1300 by then. If they never skip meals then 1500g is definitely possible. I have put on more than that in less time with the same starting size