Gouramis are tropical fish, meaning their temperature requirements are in the upper 70"s, while goldfish are temperate - room temperature is fine (high 60"s low 70"s). when out of their normal temperature ranges, fish are more susceptible to diseases, and their growth patterns are not normal.
Fancy (round-ish) goldfish can get very large. Regular fantail goldfish can get as big around as a 1-liter bottle. To be able to live and grow, they both could be put into a 55g tank. a 30g would do well until they get bigger though. The gouramis could probably stay together in the 10g, unless they beat each other up. Are they kissing gouramis? Those actually get quite large and would need a larger home accordingly.
Fuzz is not normal growth on fish. It is fungus, and you did good to treat it.
Water changes are very good for fish, and very important. Living in a fishtank is for fish what living in a airtight shed is for you - changing tank water is like turning a fan on in your shed and getting fresh air. Unfortunately, sometimes the fish get so used to the old dirty water that changing it too quickly can be harmful - like if you adjusted to breathing polluted air and then got pure oxygen. You'd want to just fall over and die. I know that the water has already been changed, and I am glad the fish are doing alright. Continue doing water changes, slowly, and in small amounts until you feel that a lot of the old water has been diluted/replaced and the gravel is free of gunk.
Instead of buying bottled water for the fish, go to petsmart (or another fish store) and buy tests kits. liquid ones, NOT strips. i mention petsmart because you can go online to the website, print out the prices of things you need, and they will match the prices in the store. a test kit with everything you need is about $15 online. test kits are important because they measure the quality of the water in the tank. fish produce waste in the form of ammonia, and there are bacteria that convert this ammonia (toxic) into nitrite (toxic) and then into nitrate, which is not toxic but has to be removed with water changes. if you test and there is ammonia, there is something wrong with the bacteria which live in your tank, because either they are not doing their job or the job is too big to handle. if you test for nitrates and, you can determine how safe the water is and if it needs to be changed. nitrates under 20ppm are considered safe, and most people time their water changes to keep the nitrates low.
why are you using bottled water?
snails are harmless to the fish, but they eat your plants and multiply like crazy. if you are averse to killing them, be careful not to feed the fish too much, as snails eat the leftover food. without extra food, they won't breed so quickly. keep the gravel clean for this reason as well.
sea monkeys? no. probably hydras or some other kind of bug. generally having bugs means the tank is not clean. if the fish were going to eat them, they would have, and you wouldn't see so many.