Like fishfreak218 mentioned I do have 8 seahorses in my 45 gallon, but I do an 8 gallon water change every Wendsday evening to keep everything in check and nitrates down..
As far as mixing species I have 3 Mustangs(erectus) 2 Sunbursts(erectus) and 3 Barbouri in the tank.. It has only been a little over a week but so far so good.. The barbs were small but they eat ALOT .. It won't be very long and they will gain some size..
They all seem to get along and all still have a great appetite, so I am satisfied.. Good luck with the tank and post some pic's, we all enjoy them, John