Good question! As a general rule of thumb, if you cannot find any explicit instructions discouraging you from posting your character in the thread descriptions or guidelines of a particular RPG forum then it is likely safe to say that you are free to post at your leisure An example of this can be seen in the Atlantis RPG area FAQ or any of the threads in the St. Mungo's RPG forum. In terms of the feast, you are welcome to have your character venture elsewhere, though some would advise that you keep one clone (one appearance of your character) out and about at one time. This means only posting at one table, then RP them leaving, then to the next table. This is not a site rule, but simply a suggestion

If you have questions for a specific area, it is a good idea to contact the RPG admin for that area or one of the existing leaders (Hogwarts staff, Ministry department head, Diagon Alley Proprietor, site mod) and they will be glad to point you in the right direction!