SPD's will work with Sidi mountain shoes just fine. A SPD pedal will have float with some the ability to adjust tension. The Crank Brothers pedals have some float also; but you can't adjust the tension.

There are platform pedals for both companies. Crank Brothers has the Mallet. Shimano has multiple platform pedals (PD-324, PD-424 and a couple of others). When truly mountain biking, I feel the platform pedal is just another part that can get caught on something like a rock or root.