I'm thinking otocinclus rather than corys; but really, determined cichlid parents should be able to fight off casual incursions from BN or corys with lots of hiding places. I've had small cichlid breeding survivors in planted tanks many times, not many fry, but some. That worked for my way of thinking & wanted survivor rates. That said I didn't try Bolivan rams...maybe they're just too wimpy vs apistos etc? That's kinda the way I hear it around the web, not to say all Bolivians are that lesser parent-wise? I dunno...
Is your tank as heavily planted as you've said? There's some debatable issues there...lots of caves, roots?