Being young he probably won't offer much protection initially and will probably get harassed by your hens. Plus side is he might learns some respect for your girls and have to earn theirs.
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You should actually quarantine any new chickens, unless you are willing to risk bringing in diseases, even with quarantine he could be carrying something your birds aren't used to and he could make them sick. Roosters sometimes turn aggressive towards their keepers, so that's always a possibility too. It might be easier to do some stuff in your yard to deter hawks. Not all hawks are willing, or wanting to eat chickens, so it could be something like a sparrow hawk, those I like.
The size is not so important imo.Bantams are too small and those I have observed do not seem to have the appropriate behavior repertoire needed. Silkies and Polish have visual impairments and the former has serious physical limitations stemming from feathering.