50 Cent was pleasantly surprised with Diddy dodging the feds' most serious charges and came up with a new nickname for his arch-rival -- the "Gay John Gotti"!!!

The backhanded compliment has its merits in Teflon ... Diddy was found NOT GUILTY on 3 of 5 counts ... acquitted of racketeering and sex trafficking of Cassie and Jane following their tearful testimonies -- all cleared off the table!!! He's guilty of 2 counts of transportation for prostitution.

50 trolled Diddy throughout the trial, even adopting the evil genius of A.I. images ... but had to tip his hat and acknowledged his OG title as a "Bad Boy" within the music industry.

Diddy still faces sentencing on the two Mann Act counts ... and he could very well be saved with a Donald Trump pardon ... something 50 is also watching closely.

Their rivalry has ranged from icy to hot for years ... and we'll know the full spill once 50's doc on Diddy is released.