After playing this game on and off for a while now I find myself trying to push passed level 15 as I left my other acct. (which after purchasing this game again remembered the password i am such a dope sometimes.) I just hit level 20 and find myself looking for more. So far I have researched that I need to complete the main storyline quests before I make any changes or move on to another class. I am not looking in to what Free Companies are like. Unlike Guilds from my WoW i am learning that they are similar.
No, you aren't needed to commit to 1 class until you finish the main storyline, you can swap without any issues at all :)