as far as I see, the extranet server should work with combination of a small business server (i.e. having extranet server in the DMZ, the small business server in the intranet). Please consider also all infrastructre / network related issues here, not only licensing issues.
But with regard to the 'unlimited user numbers', I think you are mixing up the Extranet Server license with the Information Access Server license. The Extranet Server will not allow unlimited number of users (as I understand your question, without additional user licenses), but will need combinations of session and Usage Cals.
For further questions, I would recommend contacting QT sales rep or your partner rep.