touch and go with FF's and Rays. I've had nothing but problems before with FF and Rays many years ago where they sucked on my rays....very visible prints on all my rays from the FF....so rather then risk the rays I put the FF with Aros...which was fine...
i have a 8 inch fei feing with my henlei sp and my marble. he doesnt bother them. ive heard of some stories that they can suddenly turn. i still keep a close eye just incase.
I kept a 14 inch flagtail with my p14 for over 5 years
unless you are by your tank 24/7 like i was i wouldn't risk it
the FF had times of sucking bad on the rays and other times he didn't do it
i think it would take a very sick aro or ray to get sucked to death by the FF but it will leave marks and if you don't step in fast then it may do some real damage
FF are very aggressive at food time i saw mine chase a 18 inch ST and 20 inch aros around if the grabbed a pellet the FF wanted