Rocker style light switch: do I need an isolated power supply
From the point of view of users' safety, during manual operation, a Wi-Fi controlled switch is no different from a standard switch. Hence an isolated power supply would not be required. Isolation would also entail an increase in size and cost.

You would also need to factor in the requirement of a 2- wire switch as a replacement for a standard 2-wire switch (without neutral wire). The standby current of a 2-wire Wi-Fi switch would not light up a high wattage incandescent lamp but could cause a dull glow in low wattage LED bulbs.

A metal gang plate could hinder Wi-Fi access.