Anything that is dust extracted, and can be used fo horses should be fine for chucks
, Regards the pecking, if you have tried the anti-pecking spray, and they have lots of room/things to occupy them you may have to try beak bits
Just thinking, is the dominant one deffo an hen?...when my cockerel treads the hens he sometimes pull feathers out the back of the neck. Also sometimes in the absence of a cockerel, the dominant hen 'plays' at being a cockerel, so maybe its happening because of that...either way you may want to try a beak bit