Cook 1 cup acini di pepe pasta al dente in a large pot of boiling water. This should take about 6 minutes. Drain the pasta and rinse it with very cold water. Place the cooked pasta in a large bowl and refrigerate.
Using a fine mesh sieve over a bowl, add 20 ounce crushed pineapple in juice and press down with the back of a spoon to extract as much juice as possible. Transfer drained crushed pineapple to a bowl. Do the same with the 20 ounce pineapple tidbits in juice and transfer the drained fruit to the bowl with the crushed pineapple.
Measure out 1 cup reserved pineapple juice.
Use the sieve to drain the 2 11-oz cans mandarin oranges and combine the drained oranges with the drained pineapple. Cover and refrigerate overnight. Any remaining juice can be discarded or used for something else.
Make the pineapple custard: In a saucepan over medium heat combine the pineapple juice, 1/2 cup granulated sugar, 1 large egg, 1/4 teaspoon salt, and 1 tablespoon cornstarch and whisk until smooth.
Cook until mixture comes to a low boil and thickens slightly, about 10 minutes. Remove from heat and allow to fully cool in the refrigerator.
Combine the chilled cooked acini di pepe and the thickened pineapple custard in a large bowl. Cover and refrigerate overnight.
When you're ready to make the salad, add the drained fruit to the pasta mixture. Stir in 1/2 cup shredded sweetened coconut and 1/2 cup mini marshmallows, if using.
In a separate bowl beat 1 cup heavy whipping cream and 1/4 cup powdered sugar together until stiff peaks form. Fold the fresh whipped cream into the acini di pepe mixture. Serve immediately or refrigerate.
Notes
If pineapple tidbits aren't available, use chunks or rings and cut into smaller pieces.While standard mini marshmallows work great in this recipe, I actually used dehydrated mini marshmallows (similar to what you would find in packaged hot chocolate mix). While they're not as easy to find, they add the same great flavor but with an added crunch that really compliments the recipe.