There are a lot of questions about Mist's relations, once you get your character approved and can access the Mist forum you can read... But to echo Faldred, it really depends on your character, how you'll choose to relate to followers of an enemy god. Maybe you're so devoted that you can't abide anyone who follows a god that Mist isn't fond of. Maybe you pray to Mist, but aren't even knowledgeable enough about her to know what gods she's enemy of (aside from the obvious ones). Maybe you are a fervent follower, but embrace her chaotic aspect to the extant that your friendships have nothing to do with people's dogma.
Honestly, once you get in game you'll probably find that your character will relate to people in an organic way, that's hard to narrow to simply who they worship. If you want to play a cleric it may be a bit touchier, but personally, I wouldn't worry about it for now, just make a character that you'll have fun with.