It will depend on the type of composite. You need to talk to a contractor about it. Cause different composite weigh differently. Also 6' on center is almost too wide for horizontal we do 5' or less. Resysta is a decent composite but it is not structural ao u need actual wood beams to support it.

Red is totally right for post spacing on horizontal, unless you look at Trex Horizons. This can be installed on up to 8' centers because of it's unique interlocking picket design. Not sure if you've completed your project yet but you may want to check it out. http://www.trexfencing.com/horizons