No puffers.
Your tank "could" work for a cortez stingray but I wouldn't really recommend it...here is why:
From how you have replied and the information you have shared, it seems that the ray is more like a compliment or an after thought more so than a primary player. And by primary player I mean more like the only player. Rays/sharks really need entire tanks dedicated to their care not saying that you wouldn't be able to provide this, just saying it seems like you like a bunch of other fish that aren't going to play well with a ray. So you need to think about whether this is something you really really want to pursue...
Back to the info:
I think your tank "could" house a single cortez ray...with minimal rockwork so there is plenty of sand for the ray to swim around on....you need very fine sand too btw....The reason I say "could" is that I believe you are really pushing the limits on tank size because of the way your tank is designed as a corner...normally you would want 6x2x2 as a min for a cortez more like 8x3x2 would be better....but hard to equate that to your tank, you know what I mean? So I think you would be pushing the boundaries a little but maybe not enough to make it a deal breaking...
Also your filtration will need to be rated for like a 500+ gallon system to keep up with the bioload...if you were considering large fish like the dogface puffer you might already have this type of filtration. Since alot of rockwork in the tank can't happen because of the ray most of that rock plus even more should go in your sump and you should think about getting a bead/sand filter similar to the Ultima II (you can Google that, lots of different brands out there these ones just pop-up off of Google easily, I dont personally recommend this brand just this type of filter)....
Suitable tankmates: look at Alprazo's blog, he might comment on this thread soon....
If you have anymore questions or would like to let us know how it goes please reply here
Thank you for asking questions
Zack
I also dont mean to sound rude or anything like that, I will try to be helpful if I can be