Some of the vendors like I believe Wes and maybe George at shark aquarium both had their Yarellis in tropical temps...(75-80). I'm not exactly certain of that but I'm pretty sure...Mr.Rapps also I think said he has had them in tropical water and I know for a fact that how they keep them with rays over sea's in japan and they grow huge in some good sized tanks with BD's and stuff. I think given the right set-up and a LARGE enough set up it could work... but it would not be optimum for either fish with the median temperature you'd have to meet at.
I recently bumped up another thread which had a Northern Pike successfully commed with an Armatus.... may not have been optimal for either fish, but that was one of the sickest combinations I've ever seen.