Seahorses & softies are a fine mix as long as you stick with nothing that is aggresive. So, no sweeper tentacles, no clams, etc etc. (So Frogspawn for instance would be a awful choice)
Leather's, gorgonians, zoanthids, etc etc would be great choices for a horse tank. Gives them plenty to hitch on while still being astetic. There is another post on here of someone who just posted within the past couple of days pictures of her tank w/softies & horses & she did a great job, scope it out!