Except for a few long holiday weekends - Semana Santa, New Years, it's seldom uncomfortably crowded. As for January, arrive on the 2nd and you'll find it's not bad.
If you can get a hotel room, you can find food. What restaurant said to call ahead for a reservation? Unless you have a large group or it's a very small restaurant - that shouldn't be an issue. Or are you talking about getting into an all-inclusive's non-buffet restaurant?
