Horse pellets are significantly larger than poultry pellets. Chicks are going to have trouble picking them up but an adult won't have any trouble. I was in a similar situation and had some senior feed to use up so just mixed it in with the poultry feed. My chickens picked around the pellets for the most part while the geese readily ate them. A little variety and some extra alfalfa will not hurt anything. If you ferment, that's the easiest way to hide it.
I would put it in a seperate feeder or toss it to them as a treat every now and then. I wouldnt mix it with your regular feed though cause they might throw alot of it out and waste their good food. And I wouldnt use it as their sole food source incase it doesnt meet all their nutrient needs, but Id personally feel safe giving it to them 
