Reinstall iTunes. See if that clears up those errors.
More importantly, you just received the iPod last night. Do you know its history? In order to have any chance to downgrade the iOS, you'll need to have the SHSH Blobs for the device and that specific iOS. These could only be obtained if they were stored while Apple was signing the device. That means if they don't currently exist, they never will.
Although, you're making this harder than it needs to be.
Jailbreak iOS 6.1.6 Using P0sixspwn 6.1.6 is jailbreakable. Why go through all of the effort to downgrade iOS when the version you're on can be jailbroken as it is?