Some food for thought. Keep in mind that my GMing roles overlap only very, very minimally with the ship design and combat mechanics, and so my analysis here may or may not actually be worth anything.
The way I see it, there are ship roles, and there are ship scales, and there are phases of combat. The combat phases are what most necessitates having ships of multiple scales, because only frigates can participate in Scouting and Skirmish (though there may be some exceptions to this, as the goshawnar Warhawk cruiser can also skirmish), only cruisers can enter the battle as early as Vanguard, and the size of a capital ship allows it to outfight the others when it finally enters the battle during Heavy Metal.
So. Here's a table of all the roles that can be filled, and what scale of ship I think we should want to/are allowed to fill them at.
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Frigate
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Cruiser
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Capital
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Garrison
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Yes
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Yes
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Yes
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Science/SIGINT
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Yes
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|
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Explorer Corps
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|
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Yes
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Scout
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Yes
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|
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Skirmish
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Yes
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Maybe
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Vanguard
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Yes
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Yes
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Metal
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Maybe
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Yes
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Yes
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All of our ships need to be useful while we're not at war, so every class should be well rounded enough for garrison/event response duty.
Within the garrison fleet, we also want a science-focused ship in particular to respond to Science events and do sigint missions like the T'Mir's. Since we will also need a high S and high D frigate to be the scoutships in times of war, we should make this class be that frigate.
We need a well rounded capship to be our five year mission Explorer. Any such ships that do not go to the EC can serve as sector flagships for our garrison fleet. These ships will also be our heavy metal in battle. Ditto for cruisers, only without the 5YM role.
We may or may not want to have two types of cruiser in service at one. A purely garrison/support cruiser that is not sent into battle, and a (probably more expensive) heavy cruiser that is around as good at garrison duty but also fights well, which makes up the bulk of our vanguard/metal fleet in battles. Another possibility is to have a well rounded light cruiser that fights well and can participate in Skirmish like the goshawnar warhawk, but that may require doctrine techs so probably not.
So. These roles are the ones I consider important, and some reasonable requirements:
SCIENCE SHIP: a high S vessel that can respond to science events and perform SIGINT operations. Requires: S6+
LIGHT GARRISON SHIP: a well rounded ship that can perform policing, diplomatic, and low level scientific missions in and near Federation space. Requires: D, S, and P of 3+
HEAVY GARRISON SHIP: a well rounded ship that can perform more difficult, higher stakes policing, diplomatic, and scientific missions in and near Federation space. Requires: cruiser or capship scale, all stats 3+, S, D, and P 5+
EXPLORER: a well rounded capital ship with very high capabilities to be operated by the Explorer Corps. Requires: capship scale, all stats 5+.
SCOUTSHIP: a high S, high D frigate to serve during the Scouting phase, and withdraw before the Heavy Metal phase. Requires: frigate scale, S6+, D4+
SKIRMISHER: a frigate with good combat and D stats to fight during the Skirmish phase. It may, potentially, withdraw before the Heavy Metal phase. Requires: frigate scale, C3+, L3+, D3+
VANGUARD: a frigate or cruiser with good combat stats to fight during the Vanguard and Heavy Metal phases. Requires: frigate or cruiser scale, C5+, L4+, H4+
BATTLESHIP: a capital ship to dominate the battle during Heavy Metal phase. Requires: capital ship scale, C7+, L5+, H4+.
EDIT: its been pointed out that, for the sake of any battles we have during the next few years, I should add another role that makes use of the Miranda-A.
DESTROYER: a frigate that lacks the D and S scores needed to participate in Scouting or Skirmish, but can join during the Vanguard or Heavy Metal phases to add a bit of extra firepower. This is a non-essential role that only exists as long as we have low S and D frigates that we need to keep out of the Skirmish phase. Requirements: none.
To fill these roles, we're looking at:
Science Ship: Kepler
Light Garrison: Kepler, Centaur-A, Constellation-A, Conniebee, Renaissance, possibly Miranda-B
Heavy Garrison: Excelsior-A, possibly Ambassador when and if we have enough of them
Explorer: Ambassador
Scoutship: Kepler
Skirmisher: Centaur-A, possibly Miranda-B
Vanguard: Renaissance
Battleship: Excelsior-A, possibly Ambassador
Destroyer: Miranda-A