I agree,
I've had 5+ rays, and always introduced them together, then again always had very established, mature and healthy water, i feel that mature healthy water will sustain a new ray even if they have been subject to some parasites, they will be okay for the majority of things, if you find some massive outbreak, its always treatable. give your new ray his home and hope for the best, problems are always easy to deal with with healthy water.
have fun, seriously you are lucky to be able to have a pond with accommodating ray water conditions.... i have a huge tank in my house and cannot have anything outside cause it gets -40 Celsius out here..... appreciate your fish and congrats on adding to your pond.