Etiquettes certainly exists. Thackeray used it (‘A little place with its pompous ways, small etiquettes and punctilios’) and so have some other authors, but there can be very few occasions on which it would be needed now.
Much the same applies to fishes. It is found in for example, in the King James Bible: 'five loaves, and two fishes', but the normal plural now is the same as the singular.