By little ones and big ones I assume you mean older birds and chicks...not Jersey Giants and Silkies. If you mix older birds with chicks the least of your problem is what to feed them...The older ones will snack on the little ones. Severe pecking can become an issue and death to younger ones is not uncommon. Different age birds should not be integrated until they are about the same size, then the younger ones can safely defend themselves. There will always be fighting and pecking to re establish a pecking order, you just need to be sure one little hen isn't taking the brunt of it...As far as feed goes, Starter up to 10-12 weeks, 12-20 Grower, 20 weeks plus Layer with free choice Oyster shell. Grit should be given anytime you feed outside the starter like treats, worms, etc. If they free range they'll find their own grit but I still offer it here because finding it in the winter can be tough after the snows come. Good luck and enjoy!