Well, thats the thing, if you dont have space to create a new database, then your only option is to move all the existing mailboxes on that larger DB to the other DBs and spread them out.
You can then remove the original database and move some of those moved mailboxes back ( but not all!) to a new database , but any additional space added to those other databases wont be released on disk, just in white space within the database. This will work of course, and if that is your only option, thats the one I would take.
As far as the arbitration mailboxes, moving them alone probably wont make much difference and the space on disk wont be released regardless.