From what I have read, they SHOULD have game bird feed. Ours have done well on it. they were about 15 weeks old before we merged them with the chickens and began to eat the layer pellets. They need the higher protien for faster growth.
Your older pair of Pied Pearl Greys doesn't really need the game bird feed if it's above 16-18% protein (that's about all adult/mature Guineas' bodies will utilize, they'll just pass the rest), but it won't hurt them. They can eat a regular layer feed as their staple diet, and you could mix in some of the game bird feed if you want to use it up. They should not have it if it is medicated tho, adult/mature birds do not need medicated feed.