That size tank will be OK for some and not for others. Seahorses and pipefish have a very varied size range for the different species. Make sure they are eating (don't necessarily expect frozen) before you buy them! My work gets seahorses and pipefish in fairly often, though we usually have them for months until they slowly sell.
I would suggest Riedi Seahorses, many are captive bred and eat frozen food and do well in captivity. How tall is your tank? You're going to want a tank that's at least 16 inches (40.6 cm) tall for the reidis, they get about 15cm long.
As for pipefish, I would really suggest the bluestripe pipefish. They are available near me, hopefully they are where you are too. They are absolutely beautiful fish. We home a pair at work in a 10G, they are only about an inch long atm. Ultimately, these guys get about 3 inches (7.6cm). Ours ate frozen as soon as we got them, although that may have been pure luck.
In conclusion, do lots of research! These guys are such sensitive and docile creatures.