Finally, another person my age!
I actually play violin in orchestra so I don't know much about band other than what I have shown myself--but about the 8va in strings, go ahead, use it, we get it all the time!
To your piece, I liked it for what it was, lots of chords! I thought it sounded nice, though it would obviously sound much better live--do you play in band? If you do, you should get your directed to run through your piece, you'll hear things that you have never heard before. Overall, good job on the arranging thing...and I wouldn't worry much about the copyright thing right now.