Use the inch per gallon guideline to estimate how many fish you can keep but make sure the tank's cycled first. It sounds like you cycled with fish (generaly not reccomended) and this is rarely complete within 3 weeks (usualy takes 6 or so weeks). Test your water for ammonia, nitrite and nitrate. Also, the inch epr gallon guideline does not consider behaviour or schooling requirements. For example, most gouramies are territorial, some are downright aggressive and most tetras school.
White mouth tetras? What's the scientific name for those (or post a pic)? They should be in a school (6) and you need to find out how big they grow. With 6 fish that are probably around 1.5-2" once fully grown, your tank is already half-stocked. Some tetras are nippy and won't work with certain gouramies.
Honey gouramies are a good choice. Quite hardy and only get to 1.5". They are good top-dwelling fish but need to be kept at a ratio of 2 or more females per male (or stick to only one sex with 10 gallons per male, 5 gallons per female). In your tank, I'd reccomend getting a trio (1 male, 2 females). By the inch per gallon quideline, that's another 4.5 gallons occupied.
That leaves you with a little room for some bottom dwellers. I'd suggest 4 cories (prefferably bronze, peppered, albino (usualy color morph of bronze) or blackfin as these are hardiest - stick to just one species as these are also schoolers and like to be in groups of their own kind). With each cory growing to 2" or so, that's another 8 gallons occupied and your tank's fully stocked.
If you like, in future, once your tank's mature, you can fit in a trio of platies (1 male, 2 females) in as well for extra color - but any more than that is realy pushing it.
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