Well the afrowana will eventually be way too big for a 125.
Other than that, I don't really see a problem assuming that it's big enough to be passed the 'grazing' stage. I would be a bit concerned about two things: the cichlids either nipping the afrowana's fins or eyes and the afrowana eating your cichlids as it gets bigger. I'd risk it.
I'm not afrowana expert though and I'd love to hear from someone who isn't talking out of their head.