Hi, Pixie. This can be a bit overwhelming, but you're being given some good advice here. Don't feel bad--most of us have been there, and we're here to help--but listen up and learn, learn, learn!
As for stocking: There's a website called
AQadvisor. Check it out. Experienced aquarists can often get away with stocking more than what it says, but for beginners it's right on the money. It will warn you about common incompatibility and school size issues, too.
Right now, your tank is very overstocked, and you have some compatibility issues. If it were me, I would find new homes for the gouramis, the platies, and the harlies for starters. Then play around with aqadvisor until you get your species compatible and your stocking level well under 100%. You have a really pretty setup, and it can be saved!
Different fish have different personalities and different needs, and if you pick them purely on looks or local pet store advice, you're going to end up with some incompatible species and/or wrong conditions.
Forgive me if you mentioned this, but are you testing for ammonia? Also, what do you mean by "practically nothing" for nitrites? Any measurable amount of nitrite is toxic to fish. It sounds like you added a lot of stock to an uncycled filter, and that's part of your problem.