In a small bowl, combine the tajin and sugar. Stir to combine. Add a little bit of water to one saucer. In the other saucer, sprinkle the tajin mixture in a round shape to mimic the glass that you're serving the drink in. Dip the rim of each glass into the water -- and then into the tajin mixture. Set aside.
Add the pineapple, lime and mint leaves to a large cocktail shaker or pitcher. Using the muddler, crush the pineapple, lime and mint leaves until fragrant. Add the rum, triple sec and pineapple juice (if using simple syrup or agave add it at this point). Stir or shake until well combined.
Fill the glasses with ice. Divide the drink into the glasses. Top off each drink with a splash of club soda.
Make a slit in a chunk of pineapple and lime and affix them to the rim of the glass to garnish. Float a mint leaf in the top of the glass. Serve.