It will be important to get the water parameters as this will affect how best to treat the fish.
The recommended dosage of MG/formalin is 1 teaspoon per 10 gallons of water every other day for two weeks. 25% water changes are recommended before each dose. You should start with half dosage for the rays and see how they handle it. If all is well after 3 days go full strength.
I would recommend you use Triple Sulfa (at full dosage as per package instructions)with Malachite Green/formalin as it will buffer the effects of MG in sensitive fish and allow the tolerance of full dosage. The use of Triple Sulfa with Malachite Green (or MG formulations) also provides the benefit of increased secondary infection protection which is common with Trichodina.
Something to remember is MG is more toxic at lower pH levels below 7 and hardness so a KH of 50 and GH of 100 ppm or higher is recommended it is also important to let you know as in your case Malachite Green formulations that contain zinc are also much more toxic to fish and especially to scale-less fish such as rays. If you could find a newer product like Quick Cure I would use that over what you currently have due to the unknown Zn level, however if push comes to shove it's still better than nothing.
I would do a 50% water change, remove any activated carbon if used and wait at least 30 mins after using any redox reducing agents before adding meds to the tank.
Some additional info:
http://pubs.ext.vt.edu/600/600-205/600-205_pdf.pdf