Narthok

Yes.

What are said rules / do you have a link to them? 

I don't know if it's written, but basically if someone has taken a skin that was on a public website (like PMC), you should PM them and say that it's in use. I think I heard somewhere that it has to be like 50-70% modified to be used at the same time as the original wearer.

If it's malicious, or they don't comply, you could bring it to a GM.