If you put most male and female BN together you will get babies. You will get them every month or so. The bigger/older the fish get the bigger the spawns will get as well. My final bn spawn was over 100. Usually the better pleco ratio is 2m/1fm. The reason for this is that the female can often produce eggs every two week but it takes about a month from the spawn to when the dad kicks the free swimming fry out of the cave and is prepared to spawn again.
If this is a community type tank you might have other fish which will gladly eat the fry and that would help. But usually a few will survive given enough cover in the tank. If you get spawns and have a good survival rate you will need a plan for what you do with the kids.
When I moved on from bristles to other pleco species I brought about 65 assorted size BN to CatCon and sold them to a store for a song. I was happy to be rid of about 65+ fish.
Plecos are neat fish and the bn are great algae eaters. So if you do not want to get spawns, stay with the two FMs. If you want a male for the bristles but do not wan babiers, move out the FMs. Bear in mind males do need a cave to claim. Females will use caves but do not usually claim them.
