I suppose that technically speaking, each plant will have its own set of nutritional needs, just as people do, but I know of no specific work that has identified that, so recommend a formula that is complete with all macro-, minor-, and trace elements. Do not get one with too much nitrogen, or you'll be getting great vegetative growth and probably little flowering.
As to the when to feed or not question, that's going to vary all over the map, too, but the general rule of thumb is to feed more when the plant is growing, less when it's not. If you're one of those Aussies who grows a lot of native plants, you can take clues from the seasons.
I have a varied collection, so I have stuff growing, blooming, and resting all at the same time, practically all year. I also don't have time to give individual treatment, so I feed everything a relatively dilute fertilizer solution (100-150 ppm N) at every watering, all year. I'm in Pennsylvania, but if I was in Florida - with it's stronger light levels - I'd probably increase the concentration a bit.
My suggestion is that you visit a local orchid society and ask questions of other growers, as they will have a directly-applicable set of recommendations.
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