ropefish can get to be a rather long fish, albeit very slender. i would think a 90 would be ok depending on the footprint of it, but larger is always better.
make sure your tank is covered as tightly as possible with no openings. ropefish are the houdini of fish and will find any and every opening they can. as far as oscars for mates, it will depend on the oscar. they could view it as a very long worm, and thats gonna be it. i lost a rope to an oscar, so i personally dont reccomend it. you could try a polypterus which i do have luck with oscars with, as long as the poly is much bigger than the oscar and has some girth it seems