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Craig Stark reckons you can guide with 200mm focal length. The software used for guiding can calculate the airy disk of the guide star even if it only falls across a couple of pixels, hence finding the center of that star. More important to you may be to have a bright, wider field of view with a sensitive guide camera so you can always find suitable guide stars with good SNR. (I use a 127mm f/10 SCT with a f/3.3 focal reducer giving 412mm focal length, and a DSI II camera. This gives 0.1sec between updates for guiding. 0.2sec on nights of poor seeing to smooth the seeing out so it is not chasing the seeing).
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