Can I use an old Android phone as a kitchen display instead of buying a tablet?

Yes. A kitchen display system on Android is just a paired device of the kind A screen in the kitchen, and any Android phone or tablet will do. Add it under Settings → The POS, type the six letter code into the app on the phone, and it opens straight on the tickets with no PIN. A phone screen is small for a busy kitchen, so put it where the cook can read the colours: green edge for new, amber past five minutes, red past eight. It costs nothing extra, because the yearly price covers every device. The Tables, tabs and the kitchen screen tab in the docs describes what the screen shows.