I am currently working on ClearCase and now migrating to GIT. But we need this migration in a way that all work will be done in GIT and the data will be synced backed to Cle
Regarding branches and repos, I would go with one vob == one git repo rule, since git repo is really meant to be used by single project, the same way as for vobs.
As for branches, branch names across vobs/repos should match. Take a look at submodules in git to see if this can be used in your case.
What I personaly would like to see is a mature git-cc backend, which will allow me to use git on my dev-box, while being able to sync with corporate CC repository I am forced to use.