I don't want to undercut anything that's happened or sound snooty at all. But my take on the sig situation is if he wanted to where it let him. If it were me I would have just PMd the guy and asked him to just put a credit line underneath if he wanted to wear it. I've been in situations where people have done that and it's worked out fine, usually they're happy to do so. But I've also been on the other end where people have actually tried to pass it off as their own, the first few times it happened I got pissed and wanted them to get banned from the internet. The last time it happened it just made me realize that as much of this is your own art, that in the bigger picture, it doesn't matter too much. Just make something better.
As far as the sig goes, it looks great, and personally I don't think it matters if it came from a tutorial or not. If the tutorial wasn't intended for people to copy it and learn from it, why was it made? I'm sure anyone who uses it will come up with something similar, but that's because it's meant to be that way. So from my experience, it doesn't matter if you credit the tutorial guy who made it, just as long as you learn something from it because it does look good
just my .02 on that.
Also for the watermark, you can do it, my rule of thumb was basically the opposite upper corner from the text always seemed to look the best. But I gave way to actually using a watermark a long time ago because it really didn't do much and stick out like a sore thumb. And if anyone ever questioned you making it, you can always just give em the psd for proof.
Anyways, just have fun doing sigs and it should all work out in the end