Here is the answer to my question on the capability of the SBIG ST-i guider kit according to SBIG Support:
The ST-I at 100mm FL is capable of guiding to about 1 arcsecond. This is true no matter what the FL of the main scope. The questions are (a) whether the main scope and mount are capable of tracking to an arcsecond and (b) whether one arcsecond accuracy is enough for your intended application. If you are using a DSLR with say 5.5 micron pixels and your focal length is 6x7= 42 inches, then each pixel is resoving about 1 arcsecond. So the ST-I at 100mm is tracking to the pixel resolution of your system. If you want to track to one-half pixel resolution or one-quarter, then you should double or quadruple the focal length of the guiding lens.