If you have the means go ahead and build any factions you want. You're free to use them. Owners of a faction do not automatically claim them in forum battles, but it might be nice to ask them if they want to play.
The only rule that is generally agreed upon here is:
2010 rules wrote:Everyone's the boss of their own toys
Even more important than the physical models, players can be very protective of their personal Kanon. If a player comes to the table with the characters and storyline that sustained him and his brother through a desperate childhood thirty years earlier, don't What I Say Goes them into a black hole for the sake of making the half-assed army you invented over your lunch break seem two percent cooler. Regardless of what happens on the battlefield, players are the bosses of their own storylines. If they don't feel that you're treating their Kanon with respect, your contributions will be vetoed.
If you do something awesome you can still get away with it.