Seahorses really should be kept by themselves - they are slow movers and cannot compete well for food. The damsel and clown could cause damage. The seagrass would probably work as hitches for them.
Seahorse.org is a great site to learn more about your new additions. I dont own any myself, but was considering it and therefore have done quite a bit of research. Feel free to ask if you have more questions.
EDIT: Here's a link from that site about keeping dwarf seahorses:
http://www.seahorse.org/library/arti...arfguide.shtml