I would advise (temporarily) setting up your new domain on Server 2008 R2, as I've done with my current employer's new forests.
- Set up your new forest on a (temporary) Server 2008 R2 domain controller.
- Join your Server 2012 server to the new domain, and promote as a domain controller.
- Transfer all FSMO roles to the 2012 Domain Controller.
- "Migrate" SBS 2008 to a proper domain (that supports trusts), based on Server 2008 R2, or set up a secondary DC in the SBS domain to allow trusts natively on the SBS domain.
- If you're having difficulty, there are a couple unofficial guides to "migrating" SBS 2008 to Server 2008 R2, here and here.
- Establish your trust between the two forests.
- Migrate using ADMT.
- Replace/upgrade/demote Server 2008 R2 Domain Controller.
- Raise Forest and Domain Functional Levels to 2012.
Basically, since you should always have at least two domain controllers per domain, it's not a big deal to make one of them a Server 2008 R2 server, to provide compatibility with ADMT 3.2, perform your migration, and then upgrade or replace the Server 2008 R2 domain controller to 2012. Just one extra step so you can use ADMT to make the migration process a lot easier.