A: Well, you could… But (except for mixing the two Star Trek Fluxx decks*) we don’t recommend it, actually. No matter how many versions we publish, we really don’t encourage people to play Fluxx with more than 6 players. The problem is not the number of cards. When you have that many people playing, it can take a really long time waiting for your turn to come around again, and this significantly reduces the fun of the game. If you have more than 6 people who want to play, we always simply suggest that you split the group and play two separate games. All that said, some people do choose to mix decks, or stretch the game to include more people.
Also, wholesale mixing of entire decks can raise some issues. Though we have deliberately designed all Fluxx versions to have the same back so you can mix and match cards, particularly promo cards, not all versions “play well together”. For example, in Martian Fluxx, most of the Creepers are Humans. What do you do when you mix a different deck where there are Keepers who are Humans? Are they Creepers now?
Another issue is the dilution of the Keeper to Goal balance. Although most of the Rules and Actions are quite flexible, the specific Goal set in any given version is designed to go with the Keeper set. So it’s harder to get Keeper sets that match the Goals which come up, since half the Goals only go with half the Keepers. One solution we’ve thought of, but not tried, is to use Double Agenda like a Meta-Rule, which is to say, a Rule which is permanently in play throughout the entire game. That way there is more of a chance that the Goal you need will come up to match your Keepers.
* The difference with the Star Trek decks is that they have so many analagous cards (both have a Captain, Engineer, Transporter, phasers, Enterprise, etc…) that it was just a no-brainer to make them combinable, and we even made an expansion to assist in that, called the Bridge Pack. HOWEVER, we still advise against playing with more than the six people recommended, for that simple reason first stated: it gets to be boring waiting for your turn to come around.