Depending on your budget you could either get cherry shrimp (lots of people have them on here for about $1/ea.). Or you could go the more expensive route and get some orange, yellow, or chrystal shrimp (around $5/ea.) and breed them. You could easily make your money back plus some.
A floating piece of styrofoam would be very unfair, as fiddler crabs are land dwellers that need water, not the other way around. For Pufferfan's 6.6g, the layout would need to be changed, and it'd rule out any tankmates.