Is it possible to authenticate both AD users and not AD users at the same time? (We have a website which is mainly used internally in the company and we'd like users to be authenticated using AD, however a few number of external users has to use the web-site as well, and they are not part of our AD, so the need to be authenticated using a Database lookup. )

Yes. Visual Guard support mixed mode authentication.
You can log with Active Directory accounts and username/password accounts in the same application.
You can manage username/password accounts and define your own password policy with Visual Guard console.
You can also manage these accounts through your web application, using Visual Guard Membership API.

Visual Guard .NET