Best practices for using Content Types

You are on the right track.

Yes, use inheritance.

Keep it simple, only add fields to a CT that will be needed to for metadata navigation and/or search refining and/or workflow automation.

Do NOT add fields just because you have the data and it MAY be helpful some day. It is all to easy to end up with 7-10 fields, resist the tempation. 3-5 is almost always enough.

Company should be the name of the firm that is most relevant to the document. if all of the files have the same value in the company field, this data does not provide any value because it is not unique.

The real trick to CT design, is to make the fields unambiquous so that they can be populated consistently by different people without any training. You should not have to explain to the new person what type of information must be put into a field, it should be self-evident.

Start small with just a few CTs, then expand to other business areas as needed. Many companies can easily have dozens, but it is all too easy to create a bunch of them and then decide you need to something different. Change management and rollback of CTs in use can be a huge pain.

Watch out for columns created in a list that duplicate/conflict with the fields from a CT, this can cause a lot of confusion. In general, if a field is going to be reused, it should be created as a Site Collection column (they are called Site Columns in the UI (and they can be created in a subsite) but is is almost always better to create them at the top level of a site collection (or in the ContentType Hub site)