My daughter recently got 6 "Green Fire Rasboras" that looked for all the life of me like neon tetras with green where the blue is supposed to be. With a little research, we found out they are indeed tetras.
Though four of them looked good, another two looked very skinny. Luckily they seem to be vigorous feeders, and avidly take frozen, finely crumbled flake, and Spectrum slow-sinking pellets for small fish. The best way to find out what your fish will like is to try a few different foods. Most fish go nuts over frozen food. For fish of this size baby brine shrimp, daphnia, mysis shrimp, tubifex, and of course, bloodworms. See how they do with them whole, but if they are too small to eat them whole, you may want to cut them up. The daphnia are very tiny, and easily spread over the tank. Just be very careful not to put too much in at a time, because I have found these tend to pollute the water. A good rule is to put in a little food, and if they eat it all in a minute or two, put in a little more.
At their size I would recommend about three to four very small feedings a day. One could only assume at this size that their stomachs are tiny, and because of this, cannot eat as much at one time as some other fish. Once they are looking healthier, I would drop back to two to three feeds a day, and begin to skip one day a week. In the wild, fish find food sporadically; they don't have regular feedings by a loving hand. This will encourage the fish to hunt for whatever miniscule critters may be living in the tank, as well as giving your filter time to cope with cleaning the water. It will also help prevent an outbreak of unwanted snails, which I learned the hard way.
Do keep an eye on the ones whose bellies look collapsed. If this does not improve with an abundance of food, I would begin to look at other causes for their thinness, such as Chronic Wasting Syndrome, or Neon Tetra Disease. I am not an expert on medications, but some would recommend treating them for internal parasites from the start. But I will leave this for others to discuss, and bid you farewell and good luck.
Bye for now
soul-hugger