Can the POS work as a plain restaurant cash register on day one, cash only, and grow from there?

Yes. Under Settings → The POS, press Set up cash and card in the How people pay section and you have the usual buttons at once. Open the day with a float, sell with cash, and count the drawer note by note at closing for a Z report. Leave the printer, the stations and the tables for later, because none of them is needed to sell. Add the card machine, gift cards and house accounts when you are ready, and nothing already done has to change. The POS and the counter tab in the docs has a guide on opening and closing a shift.