I have another smaller firemouth and a pleco (can't tell you what kind of pleco because the LFS had them mislabeled and had no clue; they had it labeled as a rubbernose, but it's definitely not). Within a day of being in the new tank, they paired up and became somewhat aggressive toward the third firemouth. I created some good hiding places so he can get away and so that he is not always in their line of sight, and he seems fine. Since spawning, they are aggressive toward him AND the pleco if either comes anywhere near the babies. I haven't observed an actual attacking, but plenty of chasing.
I'm still undecided about whether to add any other tank mates or to just keep it basically a single-species tank with the token pleco for cleanup duty (and because they're just so cool). Down the road, I may add another S.A. cichlid species, or may add jewels since they seem to go well with S.A.'s. Although I would love a slightly more populated tank, it may be a bad idea if these guys are constantly going to spawn, since they'll always be in aggressive mode. But, again, undecided.
The sand is just basic pool filter sand. I've definitely seen finer sand out there, but this isn't very coarse at all. They love digging and picking it up and sifting it. The pieces of gravel you see are local to that one spot and aren't mixed into the sand in general, in case that's what was throwing you. I had gravel as my substrate until a heater malfunction while I was away on vacation cooked my entire tank. I decided to start over with a medium-sized SA cichlid tank, so I thought a sand substrate would be better. But I still had a good colony of bacteria in my initial substrate (since there was unfortunately a fair amount of cooked fish matter generating ammonia, my bacteria colony continued to thrive), so when I changed to the sand, I filled up the flower pot on the far left with gravel so as to not have to re-cycle the tank (I'm guessing you were probably also wondering why I had a flower put standing up instead of on its side). The firemouths can't resist digging in it every now and then and making a mess, so some of the gravel spilled out on the sand in that area.
