As mentioned above, this is a bad idea. The angel will pick at the seahorses - any fish that picks at algae/rocks will pick at seahorses - which is why tangs and lawnmower blennies are bad for seahorses. The angel may also scare the seahorses into stopping eating, regardless of how well they are eating before the angel is added.
Please do not buy the angel if you want seahorses, or vice versa.