A ton of mainstream artists get down with underground cats. It's a manner of personal choice that he doesn't fuck with anyone not eventually hitting the airwaves or from his old crew.
Paul Wall
Ice Cube
Shawnna
Scarface
All mainstream artists on your favorite rappers last record.
8ball and MJG, J and Paul, E40, Busta, and Game are just a few of the mainstream artists off top that fuck with underground acts or push their underground counterparts into a better light. None of which has anything to do with the industry. Very few people have heard of Devius, but if it weren't for 8ball giving him a push, i wouldn't know who dude was at all.