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The imaging chip and the guide chip are exactly the same distance from the back flange, if they are both to be in focus...
At 25mm off axis the C14 RMS spot size is 60-65 um, depending on the back flange to focal plane distance (essentially what the focus knob is set too since that changes the mirror spacing). At 15mm off-axis it's about 35 um, so choose the smallest OAG you can get away with. An Edge C14 should have smaller spots than this, but it is new enough that I don't believe anybody has posted quantative results yet.
BTW a "fat" guide star does not mean you can't autoguide, it just means the faintest star you can use is a bit brighter than in a fully corrected OTA.
Noisy tracking graph? Find out what the peak to peak variation is in arc seconds, you may be looking at magnified noise from the huge image scale, or bad news, your mount is too woobly.
EB
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