Yes cheddar and mozzarella is perfect. Processed cheddar may even be preferable especially if it's the kind that comes in individual wrapped slices for sandwiches. Two slices is probably plenty but you can do more if it's larger slices of bread, you just run the risk of the cheese spilling out of the sides while cooking it. If this happens just flip the sandwich onto the cheese until it sticks to the outside of the bread. Garlic salt is also pretty crucial. Low heat and slow is the way to go it's pretty easy to overshoot and burn the bread, if this happens and you catch it in time you can use a butter knife to scrape the black part off until you get to good bread underneath.