Making the crank pulley smaller and the accesory pullies bigger will cause the accessory pullies to turn slower. Turning them slower is what free's up the HP. This means that your water pump and alternator will be turning more slowly. This causes the alt to have to work harder to put out the same voltage at idle as it does now. Also the water pump won't pump quite as fast at idle as it does now. Once your moving everything is turning quicker so your fine.
if you have some giant stereo system in the car then putting on the alt. pulley might be a bad idea. Or if your car tends to run a little on the hot side then putting on the water pump might no be a good idea. But for the most part under normal circumstances it won't hurt anything.
They can increase your MPG since they are putting less load on the engine.