Most of the
basic Source entities are there--you can't use anything from CSS, nor can you use any NPCs or weapons from HL2.
I'm sure players will be happy to see well-made maps for FF; whether they will play them is another matter.
We certainly try not to break existing maps with updates. Whether we implement a new feature is up to the interests of the individual dev and their ability to do it. We're volunteers, after all.