The five frequencies are a part of the seven frequencies shared by the GMRS and FRS radio services. GMRS is limited to 5 watts on those 7 frequencies.
I do not think there are any two-way radios available that are type-accepted by the F.C.C. in both the UHF public safety band, and the UHF GMRS allocations. If true, that would imply that fire personnel are using a separate radio on 453.900 from the one being used on the 462.xxxx channels ... unless they were using modified or non-type-accepted radios for some communications.
Unless the department has had a grandfathered GMRS license since prior to 1989, the requirement is that every person using the tac channels would have to have their own F.C.C. GMRS license, unless someone in their immediate family already had one. The license is $85 per licensee for 5 years.
I really have to wonder what liability the department and / or the county fire services would have if there was a firefighter loss-of-life during a fire that could be attributed, atleast in part, to radio problems on the tac channels. There are millions of cheap FRS handhelds in public hands, and anyone could cause interference during critical fire scene communications.