Useful to know, Carole. It's probably up to a repotting anyway, depending on how quickly the mix you were using was breaking down.
I wouldn't keep it in potting soil, though. Cymbidiums are semi-epiphytic (not terrestrial) and won't do well in potting mix (in fact, it makes it incredibly easy to overwater and cause root rot). I would minimise watering through winter and then in spring repot into some Orchid bark (Orchiata has been recommended to me by a number of growers).
When you repot it, don't divide it just at the moment, unless the plant has naturally split with all the backbulbs in the middle. I would remove 4 of the backbulbs (rough rule of thumb is to have up to one-third of Cym as backbulbs).
Give them as much light as you can without allowing them to burn in the summer afternoon sun.
Can you upload a photo of the suspected rust? I've never seen something like that on Cyms before, so I'm not quite sure what you're referring to.
