Not with those room-mates.
Seahorses are social - they do better in pairs or groups (even same-gender "pairs").
Yellow tang is not a good choice for such a small tank. He'll be OK for a while if he's really tiny, but you'd best have an exit plan for him.
I wouldn't put a seahorse in with your current inhabitants, and if you have any stinging corals, then it's definitely a no.
Jenn