Okay: setting aside the Book 5 issue for now, it seems to me that:
A)The fleet with the faster ships can choose whether or not to fight.
B) Battles are won by bringing the most guns to the fight.
C)That being said, after winning the fight, you have to be able to defend what you've taken.
D)Also, unless you're set on fighting entirely defensive battles, you have to be able to get to where you want to fight without being stopped midway.
E)Also, you need to be able to locate the enemy and deliver that knowledge to your main fleet.
Fighters are fast, and are one of the cheapest ways to bring the most guns to the fight, with the disadvantages that 1) they require retrieval, which is time consuming and 2) they lack resilience, and 3) they lack endurance. They are also limited in computer power: they're easier to hit than one might like, and they're pretty much limited to missiles in order to be effective. They're good at A, and B but of limited use for C, and no help at all for D or E.
So big carriers, alone, don't win the day. Or, they might win the day, but I don't think they can win a war.
The biggest warships are rather sluggish: if both sides are set on fighting, they can go at it hammer & tongs, but if the other side decides to withdraw in-system and fight a nuisance war, they can. 4-5000 ton ships are good at B, but they're slow (no A.) Also, they suffer almost as badly from drive hits as do merchant craft: they're fragile. Their jump range is short, too; so they're not so good for reconnaissance, and UNLESS they carry enough fuel for multiple jumps, they can be harassed on the way to their targets.
Smaller warships, of 1-3000 tons, with long ranges and high speeds, are more versatile. They can engage or disengage more readily than bigger vessels; if they carry extra fuel they have greater jump freedom. They are more resilient, in some ways, than their larger sisters (they can maintain better performance after receiving drive hits.) A ship with more than one jump's worth of fuel can hop in-system, make contact and jump out if need be. Less effective, then, at B, but A, C, D, and E speak well for task forces of mid-range frigates.
There's the additional issue of ships designed to take advantage of the Double-Fire program: ships with a power plant one number more effective than the maneuver drive: with a bigger warship, this makes for a pretty slow vessel, but puts a lot more lasers in play. The fastest of these would be 5g; Laser frigates of this kind might be even slower- meant to accompany carriers.
THAT SAID, a single ship engaging a combined fleet will be obliterated no matter what it is. So while individual cruisers might succeed at reconnaissance missions, any engagements will be done with battle fleets, and they'll move in concert. "The Fleet That Preys Together, Stays Together"
(Which means I might want to convert the "Statistical Method" of combat resolution from TCS over to Book 2 combat rules. I don't want carpal tunnel from dice rolling.)
Here's a guideline for what I want to put together for a TL 15 book 2 fleet:
Fleet jump capability of 6 parsecs, either at once or in two Jump-3s without refueling
Significant elements of the fleet should have an in-system speed of 6gs.