You can't host net.tcp under IIS 6, it supports only HTTP(s). So you are limited with HTTP bindings only (basic, ws or 2007). In order to provide net.tcp and other protocols, WAS is required. You can activate it standalone, but I advice you to install it both with IIS 7 (it is installed as a part of it), because IIS 7 gives a convinient service management platform in addition.
Another solution it so change the hosting environment to make it self-hosted or service-hosted, using ServiceHost class instance, which supports tcp protocol.