OKAY! So, I have a question: Minumum gallons for.. lets say.. four neon tetras, or four guppies, water preference on these species (salt or freshwater), lighting requirement or no, anything related to the care of these guys, would be really really helpful thanks a bunch!

Both neon tetras and guppies are freshwater fish.
If you go by the 1 inch per gallon rule, then 4 neons would require 4 gallons of water as they grow no more than 1 inch each. 
Guppies are livebearers.  Best to keep them and the neons with a heater so the water temp stays between 70-77 degrees.  Guppies grow to two inches.  Guppies like hard alkaline water so ph around 7.5.  Male guppies have beautiful colorful tail fins while the females tend to be less colorful.  I would also, feed a food especially made for guppies since the food is designed to bring out the fabulous colors in thier tails. 

Oh and another question here: Can I put some of those tetra things in with my other two fish? I mean, I wasnt planning on it until I got more plants for the aquarium, but just so I know so I dont end up with a bunch of dead fish.

hmmmm... okay.. wel ill get more plants then. just moer plants XD

No, please don't. We've seen those cute tetras, they are so tiny and fragile looking. They need an extra tank, not together with GF

Ok. Thanks once again guys, I don't know what I'd do without you I was only thinking about it because I was thinking I'll get a heater for my new tank. I didn't know about the pH though.

You dont need a glass tank.
I think your tank is just fine. 
The heater doesn't touch the side of the tank.
It has suction cups that keep it attached to the tank wall.

Today at the petshop we've seen a bunch of cardinal tetras together with a betta and a couple of catfish ? ( bottomeaters ) in a tank like ours. They all looked so pretty.
They must love the same water quality, but I wonder how they know which food is for the betta and which for them ?

I know our petsmart here feeds frozen food they thaw in big bowls and then they go around to each tank and put some in with a turkey baster   It's fun to watch and the fish just love the treat.  They also feed oranges, and lettuces.  The fish are treated really well at our pet smart and I've always gotten healthy fish from them.

Noticed the same here. Lol, sometimes I've got too "tricky" questions for them and they don't understand what I'm talking about when I'm asking how big the filter would be for the new tank, if they keep the new fish in quarantine... etc. Things that are rather simple, and needed to know! xD

You should try and get one Kat and you'd be their star employee because you could show all those id*ots up I was going to try to get one at the fish shop I go to before I got my job I'm at now but you gotta know all about salt water aquariums and fish too, which I don't know about.

i want to eventually. when Im done school maybe do it on the side to get money and what not.

I try to stay away from the pet shops unless I need something... Ended up taking home a little RED RED RED fantail with white face   My fiance named him Samurai because of the color, he's so cute :'D