I have to HIGHLY disagree with con999 on no fish ever being able to fit in a 6g- I've seen plenty of very nice pico and small nano tanks with fish in them. small gobies like trimmas, eviotas, neons, clowns, most any shrimp goby like highfins and watchmens, and Ive seen tiny damsels (less than .75") in such tanks, all though I would recommend against open swimmers.
that said, I dont have any first-hand experience with pipes or sea ponies, and the only species I can think of that are small enough to even fit in a tank that small would be bluestripe pipes and dwarf seahorses, although im about 90% sure that tank is a bit too big for the latter and about 65% sure that the prior wouldnt be able to find enough food.