BAHAHAH that PIC IS GOLD, is your GAR okay?
each ray is diffrent tho...
like if u put it in while the ray is young, it should be okay.
espeivcally if its surface fish, like the silver aro.
because the silver should be growing rather quickly too.
ive kept a 10cm silver aro, with my 15 cm motoro before with no problems at all.
but i think the best way to find out, ptu a few fish in i duno gurami's or something, and just see how it takes to them.
if they do good for about a month. id go for it.
after all arowanas and stingrays are a common combaniation.
not too sure about the oscars (seams to be a hit and a miss)
good luck mate