I have heard that you can use soured milk to help the Bacteria breakdown in a septic system instead of other purchased products. Is there any truth to this?

Urban Legend but won't hurt. Unless you have put something in the septic to kill the bacteria, there is no need to add anything. Sour milk probably contains aerobic bacteria and septics utilize anaerobic bacteria in the tank.

A "starter" is not needed for bacterial action to begin in a septic tank. Many bacteria are present in the materials deposited into the tank and will thrive under the growth conditions present.
If you feel that an additive is needed, be aware that some may do great harm. Additives that advertise to "eliminate" tank cleaning may cause the sludge layer to fluff up and be washed out into the drainfield, plugging soil pores. Some additives, particularly degreasers, may contain carcinogens (cancer-causing) or suspected carcinogens that will flow into the ground water along with the water from the soil treatment unit.
Got to be better then Septic Savior