Seahorses in a 10 gallon is a very doable idea...I have one in a 10,and it is doing quite well.So I guess that answers your "keeping them in pairs Q". The trick is getting sehorses that eat in captivity. I have seen three die slowly of starvation in my little setup,while the one ate like crazy.Just make sure(insist) that you see them eat before you even think about getting them...