Yes, the 300mm is a little big. Your 55-200mm with a Canon 500D close-up diopter would work OK, too, especially at the long end of the zoom. I know you liked the 50mm plus Vivitar converter, but I think other options would give you better image quality. Converters are usually best with longer focal lengths, anyway. Even if there is an "adapter" to use your Vivitar, you would most likely be without metering.