No, portal users do not count.

The only way a user will not be a paying user is if they have the portal checkbox checked and if the content of the Portal's group of access rights groups has been validated by OpenERP (especially for write accesses).

(Italicized text is a direct quote from OpenERP the last time I asked)

What 'validated' by OpenERP means is that they can only write to the database in ways which makes reasonable sense for portal users. Changing the delivery date of an order would likely be a reasonable change a portal user would be allowed to make.

As for the first question, AFAIK portal users don't count for "licenses".

The second question is more interesting: I have tried adding functionality to Portal users, and even with custom development (impossible in SaaS) it's not easy: some security related behaviours for Portal users are overridden from the standard in Portal specific code.

Succeeding in adding additional functionality for Portal users will depend a lot on specifically what you want to add. Just don't expect that everything you add will work, so it's a matter of trying.