Well, the first question, is which ray species do you plan to keep? They vary greatly in size, so figuring out the ultimate size of the ray will help with determining an appropriate group of fish.
Remember that rays are carnivores, and in most cases, won't hesitate to take down smaller fish. While rays can find shelter by burying themselves in sand, many fish will need shelter in order to feel secure enough to come out and do their duties.
If the tank is just glass + sand and no rock, then I wouldn't bother with many of the "cleaner" species. You can quite easily take care of cleaning the glass by yourself, which leaves the sandbed, and waste brought on by rays feeding. A ray should churn up the sandbed quite a lot, but if not, then you're going to want a robust fish that will disturb the sandbed to keep the detritus in suspension, but will also get large enough that a ray won't be able to successfully attack it. My personal opinion is that you should just take a gravel vacuum when cleaning the tank to keep the sand overturned as opposed to relying on a fish to turn the sandbed over.
With that being said....
@Jay Hemdal , what are your thoughts on keeping goatfish (Parupuneus cyclostoma or P. barberinoides) with smaller rays? I can't imagine them going after the eyes, but there's always a chance of predation.
The only other fish I typically think of when it comes to ray/shark cohabitation are other predatory fish like lionfish, or very large tankmates like tuna (which, you cannot keep in a 180).