I use flats pedals with pins on my mtn and road bike. I've found that trail type sneakers hold the pins well. Not too long ago, I had plantar fasciitis and the foot doc recommended some insoles. They are fairly stiff, never had a problem using these with my sneakers while riding my bike(s). Out of curiosity, I looked up "stiff insoles", and found this:
Insoles for Cycling – The Insole Store . Now, I'm on the thin side, around 160, but haven't had any problems with sole being too flexy.