36-year-old Guillard first ran into trouble on June 15, 2018 when the former fighter was working as a bouncer at The Ginn Mill bar in Denver, Colorado.

Cops say a man walked up to the entrance of the bar and confronted Guillard ... and that's when they say Melvin pushed the guy and socked him in the face -- knocking him unconscious.

The second incident happened just a few months later on Sept. 3, 2018, when Melvin was a patron at Bar Standard in Denver.

Cops say Guillard threw punches and knocked another man out after a dust-up inside the club ... and in video obtained by TMZ Sports, you can see the full melee break out. 

Guillard was eventually hit with SIX total charges for the two incidents ... including multiple felony assault counts.

But, we've learned Guillard struck a plea deal with prosecutors last month, and in exchange for guilty pleas -- he won't have to spend more time behind bars.

In the bouncer case, we're told Guillard pled guilty to one count of assault in the second degree in order to have the other assault charge dismissed.

In the club brawl case, we're told Guillard pled guilty to one count of assault in the 3rd degree in exchange for having the other three charges of assault dropped.

Seems it was a good deal for Melvin -- who was facing MULTIPLE years in prison -- because court officials tell us the terms of his sentencing allowed him to not have to spend any more time behind bars due to the pre-sentence confinement he had already accumulated.

Court officials add Guillard was also hit with 2 years of probation and will be required to pay some sort of restitution as part of his plea agreement.

Guillard was a pretty promising MMA fighter when he first started out -- appearing on "The Ultimate Fighter" season 2. He famously scored a victory against UFC stud Jeremy Stephens back in 2010.