Classical Tai Chi is the epitome of "Neigong"

Neigong exercises by definition need to combine "special breathing" with movement and in most cases are indicative of the "large frame" Tai Chi needing additional exercises. When the Classical Tai Chi "Compact Round Form" is done correctly, each movement no matter how small is a Neigong movement and does not need special breathing techniques...the breathing is natural and without conscious/deliberate intent. From Wikipedia: "Neigong exercises that are part of the neijia tradition involve cultivating physical stillness and or conscious (deliberate) movement, designed to produce relaxation or releasing of muscular tension combined with special breathing techniques such as the "tortoise" or "reverse" methods."  

Intro. to Compact Round and Square Forms

This Classical Tai Chi as a Neigong tunes the nervous system by using slow movement and sensitivity. Slow movement with sensitivity favors the nervous system with a more accurate and discriminating perception of the mechanics of the movement. Square form develops this. Yet, not all will have the aptitude or perseverance to develop the brains' ability to sense and correct any potential excess and unnecessary effort.

Learning "Round and Square" Forms
Classical Tai Chi as a Neigong refines the nerve signal in the body and is referred to as "Qi". Classical Tai Chi may be considered as a means to return to childhood. The Internal Discipline of the Form movements first tunes the nervous system in the body so that the complex and powerful internal movements will become natural and spontaneous. In China, a healthy and well-tuned nervous system is considered to be paramount to a healthy body. Physically, the internal movements penetrate to the deep recesses of the torso, stimulating and invigorating the organs and the circulation systems in the torso.