I dont know they age of your collection but females take the most time to build up so if you need girls start bringing them up. Thats what I would do.
oh sorry. all the females will be aroubd 2yrs 5-7mths in july 2010. and based on a 80-110grams/mth weight increase, they will reach 1500grams in april-may2010, or maybe earlier since they are now switching to rats.