Here is an article that you should probably read. It comes from seahorse.org.
It may help you out regarding some valuable info in caring for seahorses.
Whether or not your 12g nano is big enough to support these ponies will depend on the overall height of the inside of your tank not the outside.
A 12g may be ok for several ponies while they are small but, as they mature I don't think a 12g is big enough to keep them happy and healthy.
I wouldn't add any more than the two you have at this time.
http://www.seahorse.org/library/arti...sAndInfo.shtml