A group of firemouths would work, and I almost suggested it as an option. The reason I reconsidered is that with the species you have you're already starting to take up a lot of real estate at the bottom of the tank with the threadfin acaras, the winemilleri, and the red head tapajos. These three need some large, open sandy areas where they can sift through the substrate in peace. I'm waffling on just how many more cichlids you could add and still have everybody comfortable. I just did some calculations on actual area per fish on the bottom of the tank to kind of get a picture in my mind of the dispersion of cichlids across the bottom.I wouldn’t mind a group of firemouths but I was trying to avoid the aggression of ending up with a breeding pair
I think you could probably add a group of six firemouths. I wouldn't worry about the aggression from a breading pair for two reasons. First, firemouths are relatively "calm" as a breeding pair, compared to species such as jaguar cichlids, red devils, convicts, etc. They aren't likely to take over a significant portion of the tank in a tank that size. Second, that tank will be large enough that any other fish will be able to get out of the way easily. Actually, I'd be more concerned about the blue acaras pairing up, but there should be plenty of room in that tank even if they do as well.
As for your filtration. I wouldn't be comfortable with only three FX6s myself. I run two FX5s on my 225 gallon, and have not been happy. I'd suggest at least five and better yet six FX6s. Honestly, I've never even considered filtration options for a tank that large. When I first got my 225, it came with an underground filter (that shows how long I've had it, lol) which I wasn't about to use, but it took several years playing with filtration until I was happy with with what I had. During that time I regretted I'd bought it and was miserable because I was always messing with it. I was never really able to just enjoy the tank. I recommend that you take some time to research what other options and methods are out there for really large tanks. There may be far better options than the FX6s.