I'm not the best at giving this sort of advice since I've only been in the hobby for a year or so myself. But personally, I wouldn't add any schooling fish to this tank, I'm sorry. I have two tanks the same size, and they aren't large enough for the standard schooling fish that could cope with your current fish, like neons and cardinals. I plan to have some nano species of tetra called ember tetra in one of mine, but the small schooling fish tend to be at risk in community tanks without careful selection. They're small and nervous, and would be at risk from your gourami, or struggle to compete with the guppies and gourami for food, so species that are small enough to work in your size tank, are too small to live with the fish you currently have.
Do you feel as though the tank is missing something, and that's why you'd like to add more? Or is it the kind of new to the hobby urges that many of us started with, where more and more fish of different varieties are so appealing, and we want them all? Usually it's better to resist the urge to keep adding more, and to spend some time focusing on making the tank the best it can be for the fish you have. With your guppies, gourami, and a school of 5-6 panda cories, I'd say that that's a fully stocked tank, and not risk adding anything else for fear of overcrowding and creating unstable water parameters.
Then, once you're used to maintaining this tank, have a good handle on the species you already have, and water conditions like the nitrogen cycle and gH, kH and pH (the hardness and acidity of your water), if you'd still like more, then upgrade to a larger 30 plus gallon tank that could also house a schooling, mid level fish.
