As for your pistol shrimp, the odd time a crab has found it's way into my guy's burrow the shrimp has just picked it up and thrown it out. Things that crawl don't make good building materials after all! Your shrimp will probably take a few of those spare shells to reinforce his structure, I just take them back every now and then lol.
They definitely get bigger. For comparison, I managed to catch a picture of my little female blue leg hitching a ride on the shell of one of my big male red legs today. She hasn't grown in a long time and I'm hoping he's max size because that's the biggest shell I have!