Exchange 2007/2010 coexistence

I have some questions around the 2007/2010 coexistence.

For background, CompanyA (west coast) merged with CompanyB (where I am; east coast) and took CompanyA name.  CompanyA domain resources are gradually being added to CompanyB office and datacenter.  CompanyA has a 2007 SP2 server and CompanyB has 2003 server in their respective AD forest.  We (CompanyB) are planning to install a 2010 server locally in the CompanyA domain (we already have dc/gc in place) and are planning to export mailboxes to pst and import since there aren't a huge amount of users.

I've been catching up with 2010 changes since I've been using 2003 the last several years and have been reading a couple things that I wondered if it is a good idea or any issues to consider.  First, I've read in several places that the 2007 server must be SP2, though on Microsoft's Exchange deployment assistant shows it must be SP3.  I've also read that 2010 can send to 2007 but 2007 can't natively sent to 2010.  Also, when installing the 2010 server, the CAS role should be installed first.

I setup a test environment with a 2007 SP2 server and put a 2010 SP2 in there and had no issues.  I also selected CAS/MS/HT in 2010.  I created 2 mailboxes (one on each server) and was able to send back and forth without any issues.  This tells me that I *shouldn't* have any issues putting this 2010 server in production along side the 2007 server.  Co-workers have talked about separating 2010 roles to different servers.  Is this really necessary?  In large environments I can see that happening; but with less than 100 mailboxes, I would think 1 server with those 3 roles would be sufficient.  Anyone had experience doing this (2007 and 2010 coexistence) and could provide guidance or advice as to if this would work fine in my situation?  For my new server, it's a Dell R620 with 64gb, dual E5-2650 and 146gb local storage.  Databases and logs will be placed on san volumes connected over fc.