That's a converted Grandfathered plan.
Officially, the forum regulars don't know much about these plans since they've never been offered to the public.
You may or may not have tethering. The easiest way to find out is to try.
It's probably a 2.5 or 5GB cap because of the grandfatheredness (again, assuming it works).
You won't be penalized for tethering if it's not on your plan. It'll either work or not. Or it may work for a while then not. If your plan doesn't officially include tethering and it connects, T-Mobile won't go back and bill for it.
You can always try running an experiment.