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For guiding at full focal length on a large SCT - on or off axis guiders should be your primary solution. Differential flexure will kill your guiding every time if the mirror shifts or flops by even the slightest amount varying with elevation. The tolerances are miniscule - so most folks find out after a lot of pain your have to go with a guider splitting light (on or off axis) from the primary imaging train to get great results.
I use an Off axis (giant Lumicon) prism and get great results normally. Only when I can't find any guide stars (using a not too sensitive Meade DSI II Pro mono guider to I switch to a side by side (actually piggy backed side by side) 90mm refractor with a Lodestar guide camera (but that is my last result fall back - only when the OAG's camera can't find a guide star)!
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