Any fish if kept and raised alone in a well filtered tank will grow much faster than fish that are raised in a community setting. Cause then you eliminate the factors of dominance, competition for food, extra waste products, etc. The growth rate will vary depending on how you're keeping your fish. As stated, it basically depends on these factors listed in no particular order of importance:
1. Size of the aquarium. More space means faster and less restricted growth.
2. Water changes. How frequently do you make water changes? More water changes a week means much faster growth.
3. Food intake and quality. Self explanatory. However, one note, in my experience, live food seems to help promote faster growth. By live food I mean ghost shrimps and feeder fish, not worms etc.
4. Competition & hierarchy. Obviously, if there are more fish in the tank, the less food your jag will be able to get. If a fish is at or near the bottom of the hierarchy, it will have almost no visible growth as it attempts to remain non-threatening in appearance to the more dominant fish in the tank.
5. Stressfulness of the environment. I would say this factor would be an accumulation of the above mentioned. Are the parameters suitable for the fish that you want to keep? Is the aquarium located in a high traffic area that spooks the fish into hiding? Etc etc. Does it have a place to hide and feel comfortable in in the first place?
As for Johns' thing. Yikes, firemouths in a 75 with 8 significantly more aggressive and deadly cichlids spells doom for the poor fms. If you wanted a dither species in the 200 with the jag pair and devil pair as the main attractions, go with the much hardier and feistier convict cichlids. If you want something different from the convict, then try a group of neets.