Ive never kept oscars myself, but we have 17 rays.
It is awesome that they are going to have such a nice growout tank! (lol, motoros are beasts, and if fed right sometimes they get rather massive.)
I dont think oscars + motoros is such a great idea; again, Ive never done it, and this is all heresay. Ive heard some nasty horror stories about it, however. The arowana should be perfect, as well as maybe tropical gars or the like. I absolutely love stingrays, and would hate to see an oops post about this. If youve already done your research and know better, let me know! Im always open to a good lesson now and then.
BTW, bad-ass tank build! That thing is beyond cool!