In order to safely overstock any tank, you need two things: over-filtration and a lot of plants. Both of those things will help take care of nitrates and the extra waste. However, schooling fish need as wide an open space to swim back and forth as possible. Because they school, the attempt to follow each other will become confused if they have to turn around too quickly, which in turn causes stress that will make your fish sick and unhappy. If you aquascape well, you can set it up so both requirements are made.
However, as the others have said, those schooling fish really need at least six per school to feel comfortable. The plants will provide a space to hide which will reduce stress some, but mainly for the gourami and the pleco - the schooling fish, unless scared into hiding, are more interested in swimming. The problem is that if you have six fish in both schools, space becomes a huge stress trigger. I can guarantee that your rasboras will be harassing the hell out of your neons. In turn, your gourami will be upset at the commotion and that's something you definitely don't want.
My suggestion? Sell your rasboras and your pleco back to the store if you don't have another tank for them. They'll probably give you store credit if the fish are healthy enough, which you can use toward getting three more neons. I suggest the neons over the rasboras because those neons will remain smaller, which will be necessary if you need bottom dwellers. For bottom dwellers, a few small cories would be perfect. My favorite is the peppered cory, or Corydoras paleatus. They're inexpensive, beautiful and active. You'd need at least three and wouldn't want any more than that, so I'd say three is the magic number. On the other hand, you might consider otocinclus. They're very small cats that are algae eating machines (in fact, be prepared to supplement their diet with algae wafers - they are obligate herbivores). They're more often found on the glass sides of the tank or on a plant or decoration, but you'll see them on the substrate, too, especially if there's a yummy algae wafer for them to swarm on. Three would be the magic number for them, too. Pygmy cories are small enough that you could get five, but most likely your gourami would eat them.
However, maybe you'd rather not deal with a planted tank. You might still be able to handle the gourami, six neons and a few cories if you add another filter (like an internal or the Tom Mini Rapids canister filter - I have one of those and love it), and do bigger water changes. And you might want to consider two or three shrimp to help keep things clean. Clams are natural filters, but you'd only want one, because if you wound up with a male and a female and they mated, their larvae attach themselves to the insides of a fish's gills.
Anyway, that's my advice.
