I could be wrong, but a huge cichlid slamming itself right in front of you is unnerving each and every time, so I would side with the piranha remaining nervous.
I know its not a rbp..but i had a 10" RD divided in with my serra. he didnt seem to really care after the first few hours. I'd be more concerned about the cichlid somehow getting through the divider. I woke up one morning to find the divider jarred loose and about half of a RD sitting on the bottom. Just have to make sure it's strong enough to hold up to a pissed off cichlid and it should be fine