if I were to pick the Major Arcana can i use a mix of upright and reverse?
Yes. The upright meanings are the default prompts, but if you like the reverse meaning better for a specific prompt you can use that instead.
And I can do it all with different chars in the same fandom right? Not the same chars constantly.
Depends. If, in your claim, you sign up for the "general" series, then yes you can use anyone you want. But if you claim a specific person, pairing, or group, then you must write/draw whoever you claimed.