To increase growth rate you need:
1. more frequent and bigger water changes
2. more live food, with emphasis on Baby Brine Shrimp, variety and frequency
3. a bigger tank if possible
Growth development also depends on the fish's genes, but at 2 month, they definitively be larger. My 1 month old are ussually 1 in long.
I used 30 gal tanks for raising Guppy fry. I feed several times a day and cultivate my own Brine shrimp. I feed them earthworm flakes, black pearl flakes, color flakes, multi flakes, spirulina flakes, anti-bacterial flakes, anti-protozoa flakes, bloodworms, Daphnia, Infusoria and everything else I find available including green water sometimes.
The more BBS and live food with Carotene they eat, the brighter and stronger color the fry will have when they mature. So, try more WC and changing the diet. That can already make a difference.