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Having had an 8" SCT... the following assumes you have sharp eyesight.
The maximum useful magnification of SCT's is around 1X per mm of aperture, maybe a tad more - and that's in good seeing.
On most nights average seeing will be around one second of arc, worse that the resolution of your scope - so there's no point trying to push it to see diffraction rings - on most nights the seeing wil reduce the Airy disk pattern to mush.
My own conclusion was that a 9mm was optimal in the sense that images remain reasonably sharp and bright, and shows all the details to be had. Going shorter will give a larger image, less sharp, dimmer, and you won't really see any more detail.
7mm is definitely too short.
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