The triangulation is a major common concept for ALL Chinese Martial Arts.

i.e. San Jiao Bu (3 Angle Step). Basically, ALL stances should have a triangle for stability. Even contemporary wushu follows this maxim.

The turn in is more of an alignment thing...and a way to get stability through the joint (primarily the closing of the kua - hip area).

However, there are those that claim to do traditional that take this turn WAY too far. Instead of having a slight turn in, they turn the foot almost 90 degrees inward. This limits your stance, mobility (Gong bu...you would have to turn the foot BEFORE you could step), etc... and REALLY strains the knee.

While this may not be noticeable in a southern style, for a style like Chaquan, it would make the styles mobility hard to achieve. The saying there is stand like a mountain, move like a flood...hard to flood if you have to do prep to step.