Everything you do localy should be done with git (and not git-tfs) and you could do everything you want (create branch, rebase or merge,...).
git-tfs is only used to interact with TFS (fetch changesets or commit changesets).
But (because there is always a 'but' ;) ), you should NEVER modify the commits already commited to git-tfs! Just add commit to the 'master' branch and commit it on TFS...
If one day, you will use tfs branch, you could find answer in this stackoverflow question : How do I use git-tfs and idiomatic git branching against a TFS repository?