Shimano do adapters to allow SPDs to be used on shoes with SPD-SL, the SH-SM41 being the best choice with rubber treads either side of the cleat so if you want road shoes you can still use SPDs unless you manage to find a road shoe with SPD fixings, scarce as rocking horse droppings usually. SPD cleats are available on two types, a single or multi release version. I struggle with the single release on my right foot so every pair of shoes has a single release cleat on the left shoe and a multi release cleat on the right shoe.