They do grow slow, males being mature at 4 months and females at 6 months, and if their parents are large, they will be large, but the pure kinds are larger than color hybrids. The more you combine the genes the smaller they get as they revert to small wild type guppies. You can try to feed frozen brine shrimp and keep the water cleaner do lots of water changes and feed small meals 4-5 times a day, then its really just genetics that determine their size. Your water is perfect for fry growth.