A few things to add to what DuckAndDive said.
It has been shown that protein can indeed make a difference in growth rate but only up to about 40% protein content in the food. Fast growth is far more dependent on large and frequent water changes.
I find a suitable tank size for rearing a full drop of fry is at least 20 gallons unless I intend to separate the males and females as soon as they can be sexed.
Although it may take as long as 6 months to fully develop a show quality guppy's finnage, you can get a very good idea of a guppy's final color and finnage at about 4 months of proper care, including frequent large water changes. A guppy fry can easily be placed with adult guppies at 4 weeks of age with no problems if it is growing properly.
A serious breeding program does not rely on large female numbers to enable proper care. Instead a carefully chosen one or two virgin females is placed with the target male long enough for mating to be certain and is then removed to a separate tank where the drop can take place. On the other hand, hobby breeding, just to see what you will get, can indeed be carried out by simply observing a decent ratio of males to females in a mixed "community" tank.