An abnormally large male GT can attain 12", but 10" is much more common for a large male... 6~8" is more common for a female...
An abnormally large male JD can attain 12", but 10" is much more common for a large male... 6~8" is more common for a female...
12" is very common for Oscars with larger being possible...
If you keep an Oscar in a 55 gal I highly suggest you do not add tankmates.
A male JD and a male GT will clash. They look to similar and will not want to be 'neighbors' with overlapping territories. Adult GTs tend to be a little bit tougher than adult JDs of the same size/gender... So for a community tank I would psuh you toward a Dempsey over a GT.
It wouldn't surprise me at all to see a Jewel Cichlid beat up on a Firemouth, especially if the Jewels were paired off. But I've kept a pair of Jewels with a pair of Dempseys on several occasions (different pairs in different set ups) and the Dempseys pairs were consistently the more prominent force in the tank.
All in all, I do not feel a 55 gal tank is nearly big enough to 'properly' house a community of Cichlids in the range that you wish to keep. You will have much better long term success if you choose smaller growing more docile species...
When funds allow a larger tank (such as a 125 gal) stocked with a mix of Cichlids like you are considering will be a very interesting tank.