hi,
phooey is right with practically everything.
although guppies and neons together is quite a weird mix, as guppies come from very hard water regions while neons are very soft water fish. there could be some parasital problem occurring.
i'm not sure if i understand the volum right: 10g = 38 liters?
what i would do with that is 3 small loaches if you insist on loaches, one male and 2 female bettas with lots of plants.
neons and similar need plenty of swimming area, 40 liters is tiny for them. and 5 are not a school either.
another possibility would be sleeper gobies (tateurndina ocellicauda), they are very pretty, smallish and don't need too much space.
or just guppies can be quite attractive too.
or some killies e.g. 2 males and 4 females of fundulopanchax scheeli or kap lopez. they are beautiful, don't need too much space and if you put plenty of plants in, you'd also get some babies, but not es excessive as with guppies.
weird mixtures with only the optic mostly create problems. it's not just social stress that causes the diseases, also the e.g. parasite strains, that one kind can live happily with and the other kind gets ichtyo or similar (like the red indians that died of head cold when the europeans came).
less is more and also helps the balance of your tank, makes it easy to maintain and less work.
lg
niki