Did you know ... Josh Rosen has been tight with Snoop Dogg's son Cordell Broadus since the 7th grade??

It's true ... they're still good friends today (after playing together at UCLA) -- and now Cordell is giving us the REAL inside scoop on the QB. 

In fact, Cordell says critics who think Rosen would fail in Cleveland are wrong -- and explains why a BIG, FAT PAYCHECK could drive a "millennial" like Josh to greatness in LeBron's city. 

"If you give anybody $1 million they'll be mentally stimulated," Broadus told TMZ Sports at LAX. "He could thrive anywhere with the right check and the right dedication and the right determination."

Broadus says Rosen's not only a great athlete, but a great dude -- and will kill it wherever he goes.