Cook the 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 the crushed pineapple 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 pineapple tidbits and transfer the drained fruit to the bowl with the crushed pineapple.
Measure out 1 cup of the reserved pineapple juice.
Use the sieve to drain the 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, granulated sugar, egg, salt, and 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 the coconut and mini marshmallows, if using.
In a separate bowl beat the heavy cream and powdered sugar together until stiff peaks form. Fold the fresh whipped cream into the acini di pepe mixture. Serve immediately or refrigerate.