I've never kept the species but would hazard a guess that at that size they are very piscivorous. As much as I prefer to avoid the use of live feeders, I think you should be less concerned with getting them "hooked" on feeders...and should concentrate on getting them hooked on food!
I would think they would be likely to take live feeder fish, and once they are eating regularly and have put on some weight, you can then turn your thoughts to weaning them onto non-living foods. This, of course, means using quaratined or home-bred feeders, and then leaving them alone with the food fish...no prodding, tweezing, jumping around in front of the tank, herding the feeders with nets, etc.
If you wait too long and allow them to become too skinny, they will eventually pass the point of no return, at which point they will never begin feeding and will simply waste away.