need some advice

Firemouths are neat. Most of the one's I've had were pretty docile. I've never had one that was outright mean (except when breeding). You can keep them with oscars, dempseys, pretty much anything IMO. THEY will be the picked-on ones if you have dempseys and convicts. Avoid convicts, green terrors, and other really nasty fish.

How many? Loaded question...... I'd only keep one or two of each. Firemouths grow much slower than most other cichlids i've noticed, so buy bigger ones. I'd also keep a few flashy "dither" fish like hardwater larger rainbows or big sailfin mollies if your cichlids are small.

sounds like a cool tank, that's what my next tank was probably gonna be, but i decided i will keep some malawi cichlids.

As far as the information I've found about firemouths, aggression seems to be dependant on the individual fish. I've read some horrer stories, and I've read about people keeping them with guppies and platies.

As for the compatability of those fish, I have a jd and a gt. However I've only recently got them. When I did my research it seemed that jd's are a lot more aggressive then gt's. This is also the conclusion I get from observing the behavior of my fish. So personally I wouldn't recomend 2 jds with 2 firemouths. I'd say at most one jd, and maybe a different kind of fish with the firemouths. You should also keep in mind that jds will get to around 10 inches plus or minus a few.

As far as I've heard and read sevs and blue acaras should be fine with firemouths. Be careful when buying fish labled as blue acaras, as green terrors will often be mistaken for blue acaras. Blue acaras and severums are known to be less aggressive. Another plus is they average out at about 8-9 inches which would be about as large as you'd want to go with in a 75.

You could probably keep 1 of each the Blue acara and the severum in there, and 2-3 of the firemouths. I personally probably wouldn't try to push it much more then that. You could try to throw in some target fish but keep in mind with 8-9 inch fish, your target fish better be pretty big, and pretty fast or you can consider them expensive feeders. In my tank I would like to throw in target fish, but with oscars being able to push 15 inches I have ruled them out.

When you get the firemouths try to get either all males or all females. If you get a mix, they could end up breeding and then all your other fish will be left with about 10 gallons to play in. Also keep in mind that with the bulkier cichlids filtration is key.

My FM is fine with my other cichlids(parrots, cons, sajica)but hates my super red severum and it always fights with it(even though the sev is much bigger than it).

Good point Haggisman. I have read that they don't like very colorful fish, and that fm's react to the color red, so that may be why it was aggressive towards your sev.