Just to be clear (some of these are recent changes):
Transfer is indeed pooled amongst all of the Linodes under your account, no matter which location(s) they are placed in. It doesn't matter which Linodes use the transfer as long as you do not go over your total monthly quota.
Transfer over the private network does not count towards your monthly quota.
You can add additional transfer to your account by adding another Linode or for $0.10/GB under the Extras tab. Transfer overages (not purchased ahead of time) are $0.15/GB.
The initial month's transfer total is prorated when you add a new Linode or transfer package to your account. For example, if you add a Linode 360 to your account at the middle of the month you will receive 100GB of transfer with it.
Please feel free to open a support ticket if you have any specific questions.
http://www.linode.com/faq.cfm#what-if-i ... a-transfer