AquaClears are great filters, I have one too, so I highly suggest them. I'm not sure about the cover; can you post a pic? Is there a way to cut an opening for the filter? If not you may just be able to lay the hood on top of the filter (if that makes sense lol).
I believe you can use a previous SW
tank for FW
just make sure you clean it thoroughly. Hot water and vinegar would be good to use. Just rinse it very well.
Also if you need some filter media, this site
sells seeded "active" sponge filters from their fish tanks. Many members on here, myself included, have used them to help along their cycles. Just make sure you get one that says "active" next to it or you'll just be buying a plain filter. You'll also need some airline tubing (very cheap at most pet/fish stores) and an air pump to make it work.