That can be a thorny issue...
On the whole, anything that hasn't been made official in Armada or 2010 can only be used by player permission. I assume in that case you'd take as appropriate (a Nemesis carrier as a battleship and so on).
Personally, my BFG gaming is mainly done with my long-time gaming friend, and we frankly take what we want, when we want. A little loose, but that's what we like!
On the other hand we have purists who will have absolutely NOTHING to do with anything unofficial. Trying not to pigeon-hole here, but I find these people are generally from the tournament circuits (the "hardcore"), as opposed to more casual gamers. It all boils down to you opponent really.
Not trying to rub anyone the wrong way with that last line, that just comes from personal experience. I obviously got the wrong opponents!
hope that helps!