Although it is recommended to have exactly the same hardware with the same code and SEPs, so the process of redundancy would work better. Even though if you configure it on different type of concentrators, it would still work, just be aware about the maximum parameters on each device so you are not going to have issues if the 3020 fails over to the 3005 with more connections than what the 3005 can handle. I dont think it is mandatory to configure a the public interface IP address on the backup device. Following links may help you
