The spicy grapefruit base can be made up to 6 hours ahead. Add seltzer just before serving.
Muddle lime juice, sugar, and jalapeno slices, pressing on chiles and stirring until sugar dissolves. Stir in grapefruit juice. Pour halfway up ice-filled glasses; top with seltzer. Garnish with grapefruit slices.
To spike it: Add 1 ounce tequila or mezcal per serving, before the seltzer.