Slicks wont do as well as the nobbies in soft sand and mud. but grip better anywhere else. I would suggest clipless pedals and matching shoes. Clipless pedals will help you achieve the high pedalling speeds for efficient road riding (the low mountain bike gearing would also help with this). If you fall while getting used to them, it would be better to do it on your mountain bike than your nice new road bike. Bar ends set low could give you a riding position fairly close to riding a road bike on the brake hoods.