Background info: I bought a new Squier SA-100 (beginner acoustic,) my first guitar
1st Question: I seem to have on my guitar two different kind of strings, the first and second strings look like steel, and the 3-6th strings look like a bronzed or copper strings: is this normal or should I get new, better, more uniform strings?
2nd Question: My 6th string, only when I strum at forte or fortissimo sound levels, I get a buzz from the string hitting the frets (or something; that's just my best guess) how can I fix this, for cheap... or is it something that I just have to live with until I get a better guitar?
Yes, it is normal. The strings are different in all the guitars. Acoustics: it is like you describe it. If the guitar is new, perhaps you could consider to buy a new set of strings. Sometimes the guitars are so many time in the stores and they are played by many people. If they seem uniform, don't worry about that now.
On the buzz. It could be also normal. You could ask in the store (or a friend or a relative who plays guitar). If you are playing hard and the "action" is low, the string could be hitting the frets. (The "action" is the high or distance between the string and the frets.)
And welcome to GN!