That was a bad choice. The erase free space writes over the free space on the drive with ones and zeros. That actually eliminates the free space.
Do you have the original install disk that come with the computer? You're going to need to use that to boot the machine, erase and reinstall in order to get that disk space back. Make sure you have a backup of all data first.
No, it won't work as it's model specific. You're going to either need the original DVD or a retail copy of Snow Leopard. You can buy a retail copy of Snow Leopard at your local Apple store or Best Buy for $29.
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