Cut limes in half. Squeeze juice into a shallow dish or large zip-top plastic freezer bag; add squeezed lime halves to juice. Add margarita mix, next 4 ingredients, and, if desired, tequila. Whisk (or seal bag and shake) to blend. Add chicken; cover or seal, and chill at least 2 hours or up to 6 hours. Remove chicken from marinade, discarding marinade. Set chicken aside.
Prepare rice according to package directions; keep warm.
Coat cold grill cooking grate with grilling spray; place on grill over medium heat (300°to 350°). Place chicken on grate.
Grill chicken, covered with grill lid, 6 minutes on each side or until done. Serve over hot cooked rice. Serve with Mango Salsa, and garnish, if desired.