if u get a pair, they will pair up and mate....its a given. this probally would ensure the safety of the convicts but im not sure if thats all that great of an idea. oscars grow much faster then convicts, and if hes already smaller, he may end up as food. and do you really want babies? the o's will probally eat them but ya never know.
convicts are really not that agressive unless mating so they will not fight eachother even if u get 2 of the same sex
they are the same aggresiveness as oscars, but when placed with fish with much size difference it can and probally will make a huge difference.
you should be ok though. i would get another convict just because and just put out a lot of places for the convict to hide. rocks, caves, plants, etc. he'll spend lots of time there. unless the convict can fit in the o's mouth, he'll live. the o will bully him but wont do any damage.
how big is the tank btw?