Can this pass be used with any train company or just select ones?
It is valid on the routes on the map they provide, depending on which version you buy.
The companies it is not valid on are listed in their How It Works section of: https://www.visitbritainshop.com/world/britrail-gb-pass/
It is worth adding up your expected travel and seeing what inexpensive tickets are available for that travel before you spend a lot of money (a lot) in a Britrail Pass.
On the National Rail site there are lots of different area passes, might be worth checking if they work for you