Learn Russian: Они отменили все концерты, так что зал пустой. - They cancelled all concerts, so the hall is empty.

Yes, you can. Both translate to "so" or "therefore," but they have a subtle difference in focus. поэтому literally means "for this reason," highlighting the cause. так что means "with the result that," focusing strictly on the factual consequence (the hall being empty). In conversational Russian, they are often interchangeable in sentences like this.

Does the comma always go before the entire phrase так что? Could I write так, что instead?

When you are expressing a consequence ("so / with the result that"), the comma always goes before the whole phrase так что. If you split it with a comma (так, что), it completely changes the meaning to "in such a way that" (for example, "They cancelled the concerts in such a way, that nobody noticed").

Why does this sentence use the full adjective пустой instead of the short-form пуст, since it is the predicate of the clause?

You absolutely could use the short form пуст here! However, short-form adjectives often sound slightly more formal or literary. In everyday spoken Russian, it is very common to just use the full form (пустой) as the predicate, especially for simple physical states like a room being empty.

Why is the perfective verb отменили used here instead of the imperfective отменяли?