As someone who's kept garden eels for several years I can safely say that are 0 threat to seahorses. These guys are afraid of their own shadow. Sometimes they don't eat the food that passes over them because they are scared. I wonder how they even survive in the wild.
I see no reason why you couldn't keep them together, maybe the lower temp seahorses require, but I don't think it would be too cold for the garden eels.
Also after they are acclimated one feeding a day is probably enough with maybe a couple of days of 2 feedings a week. It's not so much they need to eat so many times it's that they don't always eat food since they only eat what passes over them and they're scared of the food half the time. Mine now eat PE mysis but despite the fact they've been eating it for more then a year they are occasionally still spooked by it especially the bigger pieces.