Combine the pink salt, cayenne and 1 teaspoon sugar in a small bowl, then place in a shallow dish to rim the glasses.
Juice the grapefruits with a citrus reamer or with the wide holes on a juicer to obtain plenty of pulp but strain out the seeds. (You need 12 to 14 ounces.) Add to a small pitcher.
Put the jalapeno slices in a cocktail shaker along with the remaining 2 teaspoons sugar and muddle to break the peppers down into a rough mash. Add the tequila and a scoop of ice and shake to infuse; then pour through a strainer into the grapefruit juice and stir to combine.
Slice the lime into 6 wheels and make a slice halfway into each wheel to hang on each glass. Run the lime along half of the rim of each glass and dip in the pink salt mixture, then garnish with the lime slice. Fill the glasses with ice and divide the Palomas over the top.