Prepare the pasta according to package directions to cook it al dente. Drain and rinse the pasta in very cold water. Place in a large bowl in the fridge.
Drain the pineapple tidbits to get 1 cup pineapple juice. Use more juice from the crushed pineapple if needed.
Remove 1 tablespoon of the pineapple juice and pour in a small bowl. Pour the remaining pineapple juice into a medium-sized pot and set over medium heat. Briskly whisk in the egg. Add the flour to the reserved tablespoon of pineapple juice and whisk with a fork until smooth like a paste -- add to the egg and pineapple juice mixture, whisking constantly. Add in the salt and white sugar.
Cook over medium heat until the mixture has thickened, stirring occasionally, about 10 minutes. Remove the pot from the heat, stir in the lemon juice, and allow to cool in the fridge for about 30 minutes. Pour over the pasta and return the bowl to the fridge until both the pasta and the pineapple juice mixture are completely chilled.
Meanwhile, open the cans and drain the mandarin oranges, the crushed pineapple, and pineapple tidbits. Make sure all the canned fruits are VERY well drained or your salad will be watery.
Add the drained fruit into the cooled pasta mixture and gently stir together.
Gently fold in the thawed whipped topping until all the ingredients are well combined. Cover and refrigerate for at least 30 minutes to overnight.
If desired, stir in colored miniature marshmallows and shredded coconut right before serving.