50 Cent Ringleader of Ray J and Fabolous Brawl?

We're told Fiddy had been encouraging Ray to confront Fabolous about their recent Twitter feud -- and Ray finally decided to get in Fab's face at Moon nightclub inside The Palms casino.

In the clip, you can see 50 standing between Ray and Fabolous ... with a big ass grin.

But we're told Fiddy actually jumped in to break things up when Ray J and Fab started scrapping.