well it totally depends on the personality of the fish.
i have heard some people that put plecos in the rays be totally fine although its advised against because plecos can attach itsself to a rays disk and suck off the slime coat.
on the flip side i have heard of one person who have a red asian aro with rays (thinking it was totally fine cause everyone puts arrows with rays)
and one of the rays ate half of his $2000 arowana.
so it totally depends on the personality of each fish, some things work some dont.
some are just slightly safer than others.
in the long run though, nothing will be totally safe in a ray tank, so it really is a trial and error kind of thing IMO