Exchange 2010 - Non Internet Facing CAS in Exchange 2003/2007 Environment

Hi All

I have a bit of a non-standard environment for Exchange 2010 Setup.

Details as follows.

1 x Site with 3 x Exchange 2003 Servers all SP2 (2 Hosting User mail, 1 Hosting business mailboxes/Shared)
1 x DMZ with Exchange 2007 SP3 running CAS (Used for ActiveSync & OWA for user mailboxes only).

We will be migrating the user mailboxes to Office 365 later in the year but I need to replace one of the Exchange 2003 servers (Business mailboxes) with Exchange 2010. However no mailboxes on the Exchange 2010 will ever need to be accessed via OWA or Active Sync..

I have prepared AD and meet all the prerequisites for Exchange 2010.

I have installed 1 x  Exchange 2010 CAS server as non internet facing.

I need to know the following.

1.      Will the Outlook 2003-2010 clients will connect to 2 x Exchange 2003 servers hosting user mailboxes without issue (none so far) ?
2.      Can I leave the Exchange 2007 SP3 CAS in the DMZ until all user mail is moved to the cloud ?
3.      If I can leave the 2007 CAS Server in the DMZ till clients are on the cloud can I hold off on buying a certificate and just use a self signed one?

I will then run up 2 x Exchange 2010 in DAG for business mailboxes and move the certain mailboxes over.

Thanks