Rub the rim of a 10-oz. glass with a grapefruit wedge; dip in margarita salt to coat. Fill glass with ice cubes. Fill a cocktail shaker half full with ice. Add grapefruit juice, tequila, and 1 jalapeño pepper slice. Cover with lid, and shake vigorously until thoroughly chilled (about 30 seconds). Strain into prepared glass. Top with citrus soft drink. Garnish, if desired.