Rim and tire widths
 

Hi All,
I've 3 questions about rim/tire width.

First a little bkg... for years I have been road riding Mavic Open 4 CD rims with 700 x 23c tires @ 110 psi. I recently have begun exploring light off-road trails, etc. and was looking into getting a wider & knobbier tire for loose gravel traction. I found Sheldon's rim/tire width table and wanted to know if I should base my rim size off the inner rim width or inner bead lip width?
http://www.sheldonbrown.com/tire_sizing.html#width

My own rim measurements are (roughly +-.5mm):
Inner Rim= 15mm
Bead lip to bead lip= 12.5mm
rim outer width = 19mm
(these measurements are pretty close to the rim table found on bikepro:
http://www.bikepro.com/products/rims/rimtables.html

I'm assuming I should go by the inner rim width of 15mm, is this correct? If so, I figure my possible sizes are 23, 25, 28, 32... my second question would be, has anyone experience with riding wide tires on such narrow rims?

Final question, due to my road bike tire/frame spacing, I know the wider 32 tires might not work, especially for the rear wheel, therefor I'm thinking of doing mixed width 25/28 28/32 or even 25/32. Anyone with experience riding these combos or more extreme combos with noticeable benefits to back 'em up?

Sheldon mentions mix widths:
http://www.sheldonbrown.com/tires.html#mixing

Past bikeforum thread re: pushing rim to tire widths to 35??
http://www.bikeforums.net/showthread.php?t=644