The only "temporary success" I've ever had with a P cohab was from a convict that was meant to be a feeder. Lived in the tank with my shoal for about a year and a half; until I missed one day of feeding.
Obviously everything has the high potentiality to become a meal, however, I believe it primarily depends on size.
What OP stated was he saw a large rhom with tetras cohabing. I believe this is possible only if the P is a large enough size as to where he will not bother going after the smaller meals.
I can see this as being the only semi-successful cohab.