If you had not registered your device on the find my IPhone app, but you know the serial number of that device, could you still be able to track your device

No, you can only potentially track it if you have Find My iPhone enabled on it, you can't do it via the serial number.

If this isn't hypothetical, then if your iPad has been stolen then you should report it to the police. You should also change your iTunes account password, your email account passwords, and any passwords that you'd stored on websites/emails/notes etc. and contact your carrier.