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FL ratio is just one factor. A factor of 1:5 or 1:4 is indeed a practical minimum assuming the same pixel size on both imaging and guide cameras.
In your case flexure, or more correctly, differential flexure is a potential issue. That's when the guide camera moves relative to the imaging camera so the guiding corrections keep the guide star centred in the guide camera but on the imaging camera it is moving.
At long focal lengths this becomes very noticeable as even small differences show up. The SCT has a particular problem in that it focusses by moving the primary mirror so the mirror is not fixed relative to the tube and can therefore move relative to the guide scope.
For this reason, for long focal lengths and especially for SCTs the recommendation is to use an OAG so the guide camera and imaging camera are looking at the same thing.
That of course introduces a whole lot of other issues but it does help.
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