Co-exist exchange 2007 sp3 and exchange 2010

Hi,

My current exchange 2007 use ISA as firewall and we also have edge 2007. Recently, we decided to migrate some mailboxes to exchange 2010 (co-exist). my question is.

1. My understanding I can use current edge 2007 (no plan to upgrade), just that i need HT 2010 subscribe to this edge 2007. am i right ?

2. I plan to purchase new public SAN certificate which i will apply on cas 2007 and cas 2010. Shall i apply to Edge and ISA server too?

3. We plan to do CAS array on CAS 2010. Shall I request 3 public IP (server 1, server 2, virtual cluster IP) ? or just virtual cluster IP need to be public ?

thanks in advance. appreciate your response.