50 Cent's Taking Over for Harvey

50 Cent is running the show Monday night for TMZ ... stepping in for Harvey and dropping some knowledge about today's biggest stories.

50 didn't hold back about Malia Obama's dance fest onstage at Lollapalooza, his favorite Kardashian chick, and his pal, Floyd Mayweather Jr., spending $50k just for maintenance on his rides.

And "Power" fans don't wanna miss this -- we grilled Fiddy about his character finally returning to the show.