The ST 10 working with a F6.3 reducer will give you a imaging scale of 0.89 arcsec/pixel. A bit over sampled but will not cause any real problems. You would have to go down to F5.6 to get an image scale of 1 arcsec/pixel which would be closer to ideal depending on your seeing conditions. Optec in the USA make a F5 reducer with a 17.5mm corrected circle that may do the trick (
http://www.optecinc.com/astronomy/nextgen.htm). You could also try moving the F6.3 reducer a little further away from the sensor (standard length is 105mm) to get it but I imagine distortion around the edges would be a problem. The F3.3 reducer only produces a 9mm corrected circle so is far too small to be of use.
Mark