In a large bowl, combine the shrimp, 2 Tbsp. lime juice, the garlic, and kosher salt. Refrigerate it for 15 minutes while you prepare the black bean and corn salad.
For the salad, in a medium bowl, combine the beans, corn, bell peppers, 1 tsp. lime juice, 1 tsp. oil, the salt (optional), and 1/2 cup cilantro (optional). (If you are using fresh corn, boil or microwave it for about 3 minutes before adding it to the salad unless it is super fresh and sweet, in which case you can add it raw.) Refrigerate it until ready to serve.
Heat 2 tsp. oil in a large nonstick skillet over medium-high heat. With a slotted spoon, remove the shrimp from the marinade, reserving the marinade.
Sauté the shrimp for 2 - 3 minutes until it begins to turn pink, flipping it once or twice as it cooks.
Add the reserved marinade and continue cooking for about 2 more minutes, until the shrimp is cooked through. Toss it with 1/4 cup fresh cilantro (optional). Serve the shrimp and salad with a dollop of sour cream, if desired, or refrigerate it for up to 3 days.