well normally certain cichlids can stay in with piranhas for a limited amount of time before the piranhas get either get big enough to eat them, or get hungary
ive talked to a guy who kept his rbp in with africans for life with no problems
which even tho it worked was realy dumb to do
now convicts are probobly the best cichlids to have with piranhas if you realy wanted to keep things with them
because theyre cheap, and not to agresive but not not docile
but nothing can be kept with piranhas safley
NOTHING
now lets talk about ur tank
a 29 gallon is sutible for a single rbp for life
the rule is 20g per piranha
but IMO 29/30g is best
now putting up a divider would take away so much room for a little convict
convicts ( as worthless as they are ) are one of my favorite fish
but if u realy wanted to keep it
i would just get another tank
or put it in with the piranha and within a few weeks surley it will be gone
but the piranha needs all the room it can get
i hope i helped