punctuating So and Then at the beginning of a sentence

Solution 1:

In each of the OP's listed examples involving then or so—

And then that's when you went to the store?

Then at McDonald's you were only there for a year, year and a half?

So, if we talk over each other, it won't be clear.

So, the last seven years you worked for Dollar General, correct?

—ElendilTheTall's comment (above) that including or omitting a comma after then or so is simply a style choice accurately describes the situation, because the sense of the sentence remains unchanged whether the comma is there or not.

But as Edwin Ashworth notes (in another comment), there are times when a sentence may mean different things, depending on whether the author includes or omits a comma after one of those words. In the case of so, this happens because the comma can indicate a difference between so in the sense of "as a result":

So, we spent the night there.

and so in the sense of "in that particular way":

So we spent the night there.

And in the case of then, it happens because the comma can indicate a difference between then in the sense of "it logically follows that":

Then, we can eat.

and then in the sense of "at that time":

Then we can eat.

In many instances, context will give a reader enough clues to figure out what sense of then or so the author intends, regardless of punctuation. But consistent use of suitable punctuation in potentially ambiguous situations involving these words will make the reader's job easier.