There aren't any really good tank mates for seahorses. the main reason being, is the slow rate that seahorses eat. Any tank mate, including pipe, would out eat the seahorse. Causing the seahorse to slowly starve.
Seahorses digestive system is not very efficient~ they to not have a stomach, just one long intestinal tube. That is why it is so important that they are well fed, and hence why they make so much more waste. There are lists of tank mates, but I'd start with the seahorse and wait a while.