Unfortunately, all cichlids have a strong personality of their own! Many GBRs are compatible with cories... and there's a boisterous few that don't like when ANY animal is on their side with them. I have a baby convict who loves to beat up on snails that get too close, but unfortunately the snails don't learn too well.
Do the cories have any safe space where they can go, or are they harassed unrelentingly? Ultimately, it's up to you, but I would consider returning the cories because they will indeed be stressed (and even potentially killed) by the territorial ram. Cories don't seem to learn to stay out of others' territories.
You could try a schooling fish that'll inhabit the upper part of your tank - rams don't usually bother them. Of course, just pay attention when you introduce them and remove them if they're being harassed more than just an initial annoyance.
