Where could I get help on teams for special gamemodes such as Mix and Mega, Gods and Followers, and Inverse Battle? Battle Subway does not seem appropriate, but I don't know about anywhere else to ask.
The other meta games forum on smogon is a good place to start. It'll have viability rankings, general tips and discussions about the metas.
Other than that, experience, team rating and talking to players is your best bet.