InterOrg Exchange 2010 with out adding another CAS in the 2003 side

Hi I am looking at trying to get InterOrg working with Exchange 2003 and Exchange 2010 SP1

does the PF have to be homed to a CAS server and does the CAS server have to be a Mailbox server aswell?

If i am using a CAS array and add another box as a CAS role it will be out of the CAS array and users shouldn't be connecting to it? while i am doingi these tests is that correct?

i am attempting these st eps here

http://www.msexchange.org/articles_tutorials/exchange-server-2010/migration-deployment/deep-dive-into-rich-coexistence-between-exchange-forests-part12.html?printversion

is there anything else you know about that i may be missing or may run into ?

thanks again for your help