The link you posted is my tank. Yes, I did keep my seahorses with a mantis. It was a very strange coupling but my mantis was such a wimp that he did not touch anything. I put snails in, hermits, fish, seahorse, nothing... he didn't bash anything, I rarely even heard him. Like Dane said though, I feed it a lot just in case, usually give him food once a day just so he doesn't get hungry. I would feed the mantis some big pieces of krill, and used a pipette to feed the seahorse AFTERWARDS. I didn't want the two animals going for the same food at the same time.
Nevertheless, it isn't a good idea to pair these two up... I thought that my mantis would not go for the seahorse and it didn't. But that is very rare I think, and I just wouldn't risk it with another mantis shrimp. My seahorse actually died from hydriods and I pulled him out of the tank, so now the mantis is all alone now. Anyhow, I suggest if you want both to have separate tanks for them... I had them in separate tanks for the longest time and didn't put them together until I found the mantis to be the biggest wuss ever!
....i've kept odd tankmates w/ my mantis also. But one of them finally met his demise. I've kept a shrimpfish with my mantis the last 3 months or so, but noticed his bare lil back bone in there a couple days ago. It could have died from other causes I suppose but i'm thinking it was a KO. My mantis just finished molting, and leading up to it he really pretty much didn't eat for awhile, he even tossed away squid for a couple weeks. I think this can be a problem when you keep mellow tankmates with mantis. When their voracious appetite comes back shortly after molting I think those odd tankmates that worked for so long have a better chance of getting it.
On another note though, it was cool to see my mantis finally molt, he still (peacock) had some brown on his back and it has been replaced by a beautiful green, and overall the colors everywhere are more dramatic. I fed him lots of live food leading up to it, like crabs. Good to have a sucessful molt! Can't wait to see the changes next time round. Also he seems more alert and vision seems to have improved. Pretty cool