Few weeks ago I did a request for the coexistence of Exchange 2013 with Exchange 2010.
I did follow your suggestions and now I am on the process of finishing it.
There are some issues though that I have to solve in order to be fully functional.
The target is to have remove the exchange 2003 from the domain as soon as Exchange 2010 is up and running normally.
During the installation, the "routingroup" has not been created, so do not have any routing path between the exchange servers.
Also for some reason the Exchange 2010 cannot send messages outside but only to the users that created on that server. All the settings of the SMTP are the same as the exchange 2003.
I am waiting for suggestions.