Commercial electric vehicles can be fleet vehicles. Introducing them changes the propulsion and energy arrangements within a fleet; it does not replace the need to assign vehicles, maintain them, or manage their use. The practical decision is when a battery-electric candidate can take over a particular assignment from an existing conventional vehicle.
Begin with assignments whose requirements are understood and whose charging support can be made ready. A successful introduction brings the vehicle, infrastructure, people, and service obligations together. Buying electric vehicles before those pieces align can leave useful equipment waiting for the operation to catch up.