Is it possible to set up ATA on the Domain Controller itself? This is just on a test network with about 3-5 workstations.
Sure you can. it'a called Lightweight mode.
you can read about it here:
The ATA Gateway can be installed on a dedicated server or on a domain controller. If you install it on a domain controller, it is installed as an ATA Lightweight Gateway. For more information on the ATA Lightweight Gateway, see ATA Architecture.