In a small bowl, combine sour cream and chipotle sauce until thoroughly blended. Refrigerate until ready to use.
Cut each avocado in half lengthwise, then cut in half lengthwise again. With each cut, take the knife all the way to the pit and, with the avocado on the cutting board and the knife horizontal and parallel to the cutting board, rotate the avocado carefully while holding the knife in place. If the avocado is ripe, it should be fairly easy to cut through.
Take the quarters apart and remove the pit by gently twisting with your fingers.
Peel each section of avocado.
Cube, place in a medium bowl, and mash gently, leaving some chunks.
Add remaining guacamole ingredients and mix thoroughly.
Adjust seasonings to taste and refrigerate until ready to use.
Sauté the red pepper over medium heat in oil until red pepper is tender; add cooked, taco-seasoned ground beef.
Stir in chili sauce, salt, and pepper.
When heated through, remove from heat and set aside.
On each tostada shell, layer lettuce, taco meat mixture, guacamole, cheese, and chipotle sour cream.
Choosing avocados: For immediate use, avocados should be firm but have some give when you squeeze them gently in the palm of your hand. I usually buy the Hass variety, which turns darker as they ripen (other varieties may not darken with ripeness). The color of this variety in addition to the feel when you give them a gentle squeeze will help you determine the level of ripeness. If you end up with unripe fruit that you need to use, you can hasten ripening by putting them in a paper bag and rolling down the top. Leave them in the bag at room temperature until they ripen, checking them each day with a gentle squeeze.