glad to hear you chose new worlds...you won't be disappointed. 
as for what you want to stock with...imo a convict, female dempsey and a pair (I assume you mean breeding pair) of firemouths is a bit much for a 55. the convict and dempsey will each need their own territory and the firemouth pair will also need a territory and a much larger one since they will be breeding.
if you like firemouths then how about a setup like this...6-8 firemouths, 10-12 platys and a bristlenose pleco. the number of firemouths may have to be reduced as they grow larger and pair up but firemouths have been known to do well in colonies so you should be able to keep at least 2 pairs, possibly 3 in your 55. the platys are a natural "tankmate" for firemouths and other central americans and the fry they produce will make for a nice treat for the firemouths. you could probably also do a small to medium sized catfish in your setup if you want something to pick up uneaten food, though the firemouths should do a good job because they sift through the gravel possibly moreso than most other central americans. if you go with a catfish I'd say a striped or spotted raphael may be best suited to put up with aggression from the breeding firemouths.
if you don't want an all firemouth tank, then how about a pair of firemouths and a pair of smaller Cryptoheros. I wouldn't go with convicts because they become very territorial when they get larger and are breeding and may want the majority of a 55 as their territory which won't leave much room for the breeding firemouths. also, convicts have several smaller, more colorful and less aggressive cousin species. examples include C. spilurum (the blue/jade eyed cichlid), C. sajica (T-bar cichlid), C. nanoluteus (sometimes called a "yellow convict"), C. myrnae (topaz cichlid I believe) and C. septemfasciatus (don't think I've ever heard a common name for this species). Honduran red points are another gorgeous Cryptoheros species that are comparable to the ones I've listed. You could go with a pair of any of these Cryptoheros, a pair of firemouths and still do the platys (or mollies or swordtails...platys are my fav. of the 3) as dithers and the bristlenose for algae cleanup.
as for what you want to stock with...imo a convict, female dempsey and a pair (I assume you mean breeding pair) of firemouths is a bit much for a 55. the convict and dempsey will each need their own territory and the firemouth pair will also need a territory and a much larger one since they will be breeding.
if you like firemouths then how about a setup like this...6-8 firemouths, 10-12 platys and a bristlenose pleco. the number of firemouths may have to be reduced as they grow larger and pair up but firemouths have been known to do well in colonies so you should be able to keep at least 2 pairs, possibly 3 in your 55. the platys are a natural "tankmate" for firemouths and other central americans and the fry they produce will make for a nice treat for the firemouths. you could probably also do a small to medium sized catfish in your setup if you want something to pick up uneaten food, though the firemouths should do a good job because they sift through the gravel possibly moreso than most other central americans. if you go with a catfish I'd say a striped or spotted raphael may be best suited to put up with aggression from the breeding firemouths.
if you don't want an all firemouth tank, then how about a pair of firemouths and a pair of smaller Cryptoheros. I wouldn't go with convicts because they become very territorial when they get larger and are breeding and may want the majority of a 55 as their territory which won't leave much room for the breeding firemouths. also, convicts have several smaller, more colorful and less aggressive cousin species. examples include C. spilurum (the blue/jade eyed cichlid), C. sajica (T-bar cichlid), C. nanoluteus (sometimes called a "yellow convict"), C. myrnae (topaz cichlid I believe) and C. septemfasciatus (don't think I've ever heard a common name for this species). Honduran red points are another gorgeous Cryptoheros species that are comparable to the ones I've listed. You could go with a pair of any of these Cryptoheros, a pair of firemouths and still do the platys (or mollies or swordtails...platys are my fav. of the 3) as dithers and the bristlenose for algae cleanup.
I found that combo over at cichlid-forum in the archives, where it "passed". 
