I have managed it before with female guppies though not males and that was mainly because i completely forgot the guppies were in there!
I had 260L that was so heavily planted that despite having 80+ fish in it, i rarely saw them all lol. Was never really sure how many cories were in there but i put 2 male endlers and 4 female guppies in there purely to provide food for the kribensis pair
It was litterally like a hedge in a box...
But even then, the male fighter was a bit special, he lived in the cave with the male kribensis and kicked out the female. The female guppies were ok but the male endlers chased the fighter on the occasion that they saw him.
I think even if they do get on, its cruel because its creating an unhappy and stressfull environment for the fighter.
I also always found it was the fighter that caused the aggro (guppies just wanted food and females!) but the fighters being so over bred with poncy tails couldnt man up and finish the fight they started and ended up with nipped tails.
I would always refused to sell fighters and guppies to the same tank. No matter what the persons experience in keeping them together was.
I dont spend every day cleaning and feeding and caring for the fish just to sell them to an unhappy and possibly dangerous home.