"Not good for an agency to be armed with an assortment of different firearms (training and sharing weapons and ammo require uniformity)"
Documented cases of officers needing to exchange ammo in a gunfight are rare. I can't think of one off hand. I know that if I am in a shootout, and another officer has fired up all his ammo without solving the problem, I am sure not going to give him any of my small, on-body supply so he can piss it away too!
As for department logistics, it just depends on how much an administration values individual proficiency with the handgun. No single handgun fits everybody. People who have a choice in what they carry tend to shoot it better than a gun they were issued and may not like. The department I retired from had about 450 armed. We elected a sheriff who listened to his deputies about guns and went from a .38/.357 only agency to one that allows 9mm, .40 S&W, 10mm, 357 Sig, .45 ACP, and .380 and .38 Special for back-up and off-duty. A deputy can choose from several issue handguns or provide his own from a fairly long list.
It takes a range and armorer staff, backed by an administration dedicated to this to make it work, but if my department can make it work, I believe any department could. If a department needs 50,000 rounds a year, divided between duty and practice/training ammo, buying it on state bid, it doesn't cost much different if it is all 9mm or split up between 6 calibers.
At least, in our scenario here, in 1958, the choices were much more limited. We are a lot better off now.