To begin, gather all but the last two ingredients so that you have everything close at hand on the counter or table. Drain and reserve the juice from the pineapple tidbits, crushed pineapple, and mandarin oranges. Place the fruit in a lidded container and refrigerate until needed, later in this recipe.
Add 3 quarts of water to a large saucepan, plus 2 teaspoons salt and bring to a boil. Add acini di pepe pasta and cook uncovered for 8-10 minutes, until al dente. Drain the pasta and allow to cool.
Prepare the Custard
In a medium saucepan, combine the reserved juice from the pineapple and mandarin oranges. (This should equal about 1 1/2 cups.)
Add in the granulated sugar, large eggs, lemon juice, salt, and cornstarch.
Whisk to combine and cook on medium-low heat until thickened, stirring constantly.
Once the mixture thickens, add in the cooked pasta. Place in a covered container and refrigerate overnight.
Assemble the Salad
The next day, add the reserved pineapple tidbits, crushed pineapple, and mandarin oranges to the custard mixture.
Gently fold in the whipped topping and miniature marshmallows until all is incorporated. Cover and chill until ready to serve. Serves 10-12.