HI,
Yes you can switch to quarterly billing at any time by opening a ticket on our support desk at tchhelp.com and submit it to our billing department.
All of your new accounts will be billed on the first of the month with the exception of the sign up payments.
As far as having each account on a different server I am not sure on that as accounts are put on the server available at that time. If you put that you would like them on a different server we can certainly look at that when the new account is created and do our best to accommodate you.