You need to head over to the sticky thread fo rmore information.
The overflow box is what starts the water flowing from your main tank to the filter/sump below.
As for critters, there are many to choose, so choose wisely.
Hermits, Snails, help but some others are sand cleaning specific, like conches, cucumbers, nassarius snails, stars. But they need to be compatible with your tank.
read, some of the sticky threads and you will know what this is all about.