There is no correct use of "and so" in Standard American English. Using "and so" may be colloquial, or an empty phrase that adds no value to an idea.

Examples

Colloquial "and so"
Bob enjoyed the movie and so did Helen.
Revised
Bob and Helen enjoyed the movie.

Empty phrase
Heat causes a physical change in protein structure, and so, egg whites harden when cooked.
Revised
Heat causes a physical change in protein structure, therefore, egg whites harden when cooked.
More concise
Egg whites harden when cooked because heat causes a physical change in protein structure.

To be more concise, try thinking 'if - then', 'if - else', or 'while - occurs' instead of 'if - then - so'.