Preheat your oven's broiler. If you have a pizza stone, place it in the oven while it preheats.
In a food processor, combine the avocados and cilantro. Puree while adding lime juice and salt to taste. Cover, and set aside.
Slice the garlic clove in half, and rub the cut side onto the tops of the pizza crusts for flavor. Brush both sides of the crusts with olive oil. Spread the avocado mixture thickly over the top of each crust, then arrange chicken and tomatoes on top. Sprinkle with cheese, and season lightly with cayenne pepper. Place pizzas on a baking sheet if you do not have a pizza stone.
Broil for about 5 minutes in the preheated oven, or until the cheese has just melted and the crust is lightly toasted.