These sumps are designed with aquarium use in mind. with all the water trickling down from the bio balls and if there was use of a protein skimmer in that middle section, you can bet there are a lot of bubbles flying around in the water. That divider and sponge keep those micro bubbles from finding their way into the pump and sent flying around your aquarium. it also keep stuff like bags of carbon or heaters or whatever from getting sucked to the pump intake.
I would use all 3 if it was me.. your order should always be floss / bio / chemical. and get the biggest bag of carbon you can buy
also.. turtles don't stay 2". your smallest species is a bog turtle and they are 4-5" full grown. sliders get up around 8-10"
if you are going to DIY your sump and use dividers, I'll let you know right off to make sure the dividers are glass. if water builds on one side of a plexi divider, the thin plexi flexes and pulls on the glass and your tank will bust...