I'm sorry if I sounded a little condesending there with the "Have you done any research on convict cichlids?"
Just shocked me when you said 20... haha I was expecting maybe 4 at the most.
I would recommend you find a commited place to rehome the ones who dont pair
before you commit to buying these cons as Im sure what happened to the lady who your buying them from, could happen to you (cons start breeding and then you have more babies then you can imagine).
Cons stay relativily small compared to other members of the cichladae family but they still have all the aggression... so with 2 pairs in the 55G tank, you could be asking for trouble if both lay eggs at roughly the same time... major battles. So I would recommend only one pair.
Ox
EDIT: didnt even read the last part of your post. To tell if a pair has formed, you will see the two swimming together around the tank and will do swimming circle, in which they will lip-lock (the male showing he is strong enough and has good genes). Also, you will see the two cleaning out a "pit" in the substrate or cleaning off a piece of slate or a clay pot which will be wear they will lay their eggs and raise the young.
For the sexes as well... males are usually larger with pointier tips to its anal and dorsal fin than female. and the female will develop a orange coloration to her belly.