hardware store 10 gage electrical wire
Not sure what this is exactly but when you say "electrical wire" I will assume insulated copper, either stranded or solid.
I have tried 10 gage wire,
probably more than large enough for most typical run lengths
one wire for - and one for +
so far so good, as long as the actual conductors don't touch each other as previously mentioned
I just tried it out and it sounds good
ahh....that is the key, if it sounds good then save your money by sticking with what you have, and don't sweat it just because it didn't come in a package that said "audio" on it. There are folks out there who are perfectly happy using outdoor extension cord in 14 or 16 awg sizes as speaker cable, so I am sure your wire is fine. Don't change anything just because your wire was purchased in a hardware store.
wasn't sure if this could harm the system
The answer is "no," unless the wires become shorted.