Hello!
I prefer natural means of filtration. Some people use canister filters etc etc, but I use good old live rock, a nice 3 inch sand bed, macro, protein skimmer, & water changes!
Polyps are a fine choice for a seahorse tank, if you want to add anything else, make sure there is no aggression. (ie, no sweeper tentacles, nothing that will close & eat a horse etc etc) The only way they affect filtration is with an increased bio-load.
Some other choices, gorgonians, leathers, mushrooms etc etc. (Although gorgonians usually do not fair well) (But horses LOVE them for hitching) You could most likely get some nice replicas that will be allot less maintenace & they won't die!
Params should be kept right where you would keep for a regular tank, although, due to the sensitivity with them, you should keep your salinity a tad lower than a reef tank (I keep mine at 1.026)(reef tank, not horses) & that keeps them from catching everything they come into contact with. 1.022 is a pretty good number.
Also, be species specific. You cannot really add any other fish, well you shouldn't.
Current can be good, just not blowing them around in circles. They do enjoy to "gallop" through current, so I would recommend maybe doing an area with higher flow than the rest of the tank, it is fun to watch them play in it! =) That you will have to tinker with & adjust when you get your babies.
Also, do allot of research on the species you are going to go with. And the most important thing I will stress: GOOD, HEALTHY HORSES ARE OT CHEAP, DO NOT BE CHEAP WHEN YOU BUY THEM OR YOU WILL REGRET IT LATER. Especially when your poor horse dies from some mysterious illness. Buy well taken care of CB ponies & it will be a decision you won't regret.
(Also, make sure they are eating fozen when you purchase them, live foods get expensive!)
If you have any specific questions, please feel free to ask! =) Heather