I think alot of it has to do with size.. when they are small they are very shy, and very timid eaters.. stress out from lots of simple stuff.. Like most rays, when they are larger, they get an attitude and become more aggressive when they are comfortable with their surroundings.
As far as other ray tankmates, I would think some of the more laid back rays like Tigers and Flowers would do good, but obviously max size and growth rate make it constraining..
I always wondered if Antenna rays could eat and compete with Retics.. would be cool to have all the long-tailed species together (retics, tiger, antenna).. but the latter 2 get huge.. so.
I would stay away from any type of Motoro or Motoro variant, Castexi, or even maybe Black rays (depending on size).. as most of the aggressive eating rays will stress out the retics.. Not much first hand experience with Hystrix or Scobinas, but I bet they would work, and likely be best candidates due to small size.
It's really trial and error and you're rays own personality.. size is huge. Most people are working with retics 10" or less.. I believe when they get 12-14" they can live with most any other ray as long as it's not too much larger.