What EQ range to keep for Lead Guitars in the mix so they won't interfere much with other tracks of the metal song?
For lead guitars I tend to high pass really high up (sometimes 200Hz) and sometimes they get a boost around 3kHz. Really depends on the source sound and the mix you're trying to fit it in though.
Add a +18db boost at 1khz. That'll make sure your guitars poke through EVERYTHING!
I'd be wary of boosting 3k if there are vocals as well. Anything which is likely to battle with vocals I'll usually cut in that area.
If you've got some cool filters (ssl filters are amazing) then filter pretty hard... 200 and 8k maybe. Probably wouldn't be doing that if the filters don't sound right though. Other than that I don't have any set rules for EQ'ing lead guitars.