I wanted to ask why you are mixing grain or anything else with a complete feed and what grains.
You're adding more fat. Grains don't add muscle. It takes protein to make muscle.
The Flock Raiser is balanced. Grains are lowering the crude protein of any complete feed. More importantly, grains are much lower in some essential amino acids.
As a different species, guinea fowl don't have identical nutritional requirements of turkeys. Guinea crude protein/amino acid requirements drop lower much earlier than turkeys as they mature.
https://www.researchgate.net/publication/203330598_Nutrition_of_Guinea_Fowl_Breeders