It kinda depends on how long your Dempsey has been in the tank alone. How big is the Dempsey? Firemouths can be tenacious little s***s, but will not battle or stick up for itself if the Dempsey is much larger and is constantly going after them. If the Dempsey is not to big yet, throw in at least 2 Firemouths of decent size. 2 of them will stick together.
As for the Jag. Same thing, but from my experience on jags, one jag will hold his own. As long as the jag is not considerably smaller than the Dempsey.
Question for ya. Why a choice between Firemouths and Jags? Why not a Red Devil, flowerhorn, green terror, texas, carpintis, etc.... There are so many fish out there.
If you just like the FM and Jags, why not buy them both? 2 FM, and a Jag. But really need to know the size of the Dempsey.