I know that you can keep starfish and urchins with a cuttle but could you keep a seahorse? is there some sort of fish that is large enough not to be eaten, but small enough not to frighten the little guys?
Not usually. My experience with (non Ceph) fish is they'll eat anything that fits into their mouths, and from the posts I've seen on here so far, it seems most Cuttles will even eat things larger than themselves.
There is one post on here of a Cuttle who has become friendly with a Goby in the tank, but that seems to be quite rare.
My understanding is (other than the species you mentioned) Ceph tanks pretty much need to be dedicated tanks.
I'm sure someone will correct me if I'm mistaken, though.