When it comes to personal mules, the rules are pretty simple, really.

1. Don't have more than one mule.
2. The rules apply to the person not the account, so if something would break the rules if you did it on your main, it still breaks the rules if you do it on your mule or part-on-main, part-on-mule.

(Basically, you still can't hold conversations with yourself no matter if you do it from one account or two; the same rules for double-posting apply even if it's done main-mule or mule-main rather than main-main, etc.)

(Now, charity mules and other staff-permitted third accounts...the rules there do get a little bit more difficult)