The other thing to consider is that a small tank is going to be a much less stable system, so greater risk of problems with water stats= problems with sensitive fish like neons.
There are small fish you could fit in, pygmy cories for instance, or some of the dwarf rasboras. Or some small but hardy tetra might just work, depending on level of activity. Or a small group of male endlers.
If this is the only tank you have available you are not going to be able to both breed and keep tetras in it; that would require a grow-out tank for the fry.