i would go megaladoras irwini, or pimelodus ornatus. id sharks tend not to do so well with cichlids. also maybe a pair of suncats instead. i have a pim. ornatus and love him, they are the coolest fish, get around 14 inches max in captivity too. if you know you will have the pond in a few years(like 2) then go ahead with the irwini, but be carefull, they might grow fast.
as for predatory and ornate pim is like a mini rtcxtsn, very predatory, mine ate a dozen tetras his first day in the tank, like chase them down and eat them no sit and wait