My brother often visits different forums and he finds a ton of really cool sigs/avatars on those forums, so i was wondering if I could use sigs from different forums if i obtain permission from the owner of that sig in the forum

I don't find anything wrong with it, But its still better to make ur own

So long as they aren't advertising another forum or contain anything that is against the rules i.e. cox. Then it should be fine.

Sig quotes/google avvies are ok.

But as murm said, a sig like: "OMFG WoW IS SO COOL! JOIN IT!" is bad.