The Canon EF12 II and EF25 II work fine with EF-S lenses and the
Rebels. See near bottom of the chart at the following link. (The EF12
II does not have 6 elements. The "6" means to read note *6.)
http://www.usa.canon.com/app/pdf/lens/EFLensChart.pdf.

The Kenko extenders (the ones I tried anyway) don't work with the EF-S
lenses, but might possibly work in combination with the EF12 II if the
latter is on the lens. You would do that if you wanted more
magnification than the single tube gives.

Instead of extension tubes, consider using a Canon close-up lens like
the 250D. It is a 2-element apochromatic design. They are not
expensive and with them, you don't lose light like you do with
extension tubes.

KS