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
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...