Hmmmm… the competition must be pretty bad if roll–>flash kick is a viable tactic. However…
You have options!
Throw comes to mind… but if your timing on this is not there…
Sweep should not be an option, this is too unreliable depending on the distancing of the roll. Likely, guile will roll behind your sweep leading to a free flash kick.
As Mummy-B said, jam on cr.jab or cr.short. If you do this, every roll attempt will end up eating jabs. They may not be damaging, but the point is to get the opponent into the mindset DO NOT ROLL… EVER! Take this out, what have they got left?
Obviously, the best option is to jab/short until one hits and then follow up with the links>>>
Iori - cr.short, cr.jab X rekkas
Ken- cr.jab, cr.fierce, fireball (or super!)
Or, how about doing nothing! Land a free combo upon flash kick recovery. You might end up getting thrown tho, so use with caution.
All told, this equation is definitely in your favour, the risk/reward ratio is just not there for Guile… a flash kick that might hit versus a combo or throw that will hit.
Here’s a tip, if you’re in C-Groove, try empty jumping (jump without attacking) once in a while. You will air-block the flash kick, and can punish Guile on recovery. Use this with caution too, Guile might start using cr.fierce as an anti-air, you can’t air-block this.
hope this helps