which tank are you going to put them in? is that the 55 gallon with the dempsy, green terror and what appears to be an oscar or two? i dont think i'd put any little fish in there...
other than that... well, "happiness" is a funny term when referring to fish - i mean, you're not gonna have to dose your tank with paxil if there's only one. they're schooling fish by nature, they like to be with their own, so if you keep a few, there's more of a chance you'll see more natural behavior and thus have overall healthier fish. ive read of them being kept alone as well, so it wouldnt be as mean to house just one as it would be to house just one neon tetra. the way i see it, youve got a small school of clown loaches, why not get a small school of yo-yo's as well. i have 4 in a 46 gallon tank, which i feel will be sufficient for them. you're tank specs say you're going to get a MUCH larger tank at some point, and i assume you'll move the clown loaches into the larger one, so why not give that left over 55 to the yo-yos who will be fine in tank that size? in the mean time, you can keep the two schools together while they're all still very small, as long as you really go ahead and get a larger tank...