Cook the acini de pepe pasta according to package directions, cooking to al dente. Drain and rinse the pasta in cold water. Place in a large bowl and put it in the refrigerator.
Meanwhile, drain the pineapple tidbits and crushed pineapple making sure to reserve the pineapple juice. Set aside 1 cup of pineapple juice.
Pour the 1 cup of pineapple juice into a medium-sized pot and whisk in 1 tablespoon flour. Whisk in the egg and then set the heat to medium. 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 refrigerator until both the pasta and the pineapple juice mixture are completely chilled.
Meanwhile, open the cans of mandarin oranges and drain. Make sure all the canned fruit is drained really well.
Add the shredded coconut and mini marshmallows. Then gently mix in the pineapple tidbits, crushed pineapple, and mandarin oranges into the cooled pasta mixture. (Saving some mandarin oranges and marshmallows for the top of the fruit salad.)
Gently fold in the thawed whipped topping until all the ingredients are well combined. Cover and refrigerate for at least 30 minutes to overnight.
When ready to serve, top with more mandarin oranges and mini marshmallows.