Fleets should be built of hetrogenous groups of ships which each take a specific role within the fleet.
Why do you assume / require that all fleets have more than one type of ship?
Usually, a fleet will be built around one (or more) large ship(s) which pretty much define the Role of the fleet.
Why exclude the possibility of having a non-ship-focused strategy, in which an empire doesn't build any large ships due to focusing on other things?
But each ship has a Role with in the fleet, and it will define how that ship behaves in combat. In addition, a fleet should have an objective, even if it is to hold and observe. This will define how the fleet will react to new threats, and can make handling epic battles much easier.
You don't seem to be acknowledging any purpose for ships other than fighting other ships. What about carrying and deploying and supporting ground troops? What about map area-effect projection ships (social effects, production bonuses or penalties)? What about specialized planetary seige / bombardment ships? What about spy ships? What about economic raiding / pirate ships? What about research, industry, mining, or farming ships or ships that just hold population? Not all necessarily should be included, but not all necessariliy won't be.