Let us start with the assumption that there are approximately 1,010 ships in a Sector fleet (+/- 10%).
With 1,010 ships per sector then there should be about 63 ships per subsector.
If the fleets of the subsector could be built around either the Battleship Configuration (BatCon with 1 to 3 Battleships and their support ships) or the Cruiser Configuration (CruCon with 6 to 8 cruisers and their support ships) Or Dreadnaught Configuration (a Tigress Class Dreadnaught with their support ships) What other support ships would be included in each subsector fleet?
Would there be a mix of tech levels? (12 to 14) (one TL12 Battleship and 2 TL 14 ships?)
Would there be a mix of jump capabilities?
How many escorts and logistic ships would be included?
Would each fleet have some form of a fighter/carrier capable ship included?
How many of each kind of ship would there be in the subsector fleet?
Would the fleets in contested subsectors be heavier and lighter in safer subsectors?
Would the make up differ greatly from year 900 to 1100?
What are you trying to achieve? A setting where players can crash squadrons into each other, or an emulation of a given sector's likely naval force - and what sector? Answer to that results in different answers to your questions.
"Would there be a mix of tech levels?" Not in an emulation, at least not in Imperial space under the ship design rules currently available. Tech level is a major advantage in all of them, and the Imperium has been at TL15 for a rather long time, certainly long enough for the TL12/13 stuff to not only be retired but scrapped or turned into museums.
Below Imperial Navy level, depends. Easiest way for the Imperium to retire ships is to "sell" them to the colonials. MT Rebelllion Soucebooks mentions colonial squadrons being "equipped with obsolescent (but still serviceable) ships." That to me implies that the TL14 stuff drifts down to the colonials - which is consistent with the way the
Fifth Frontier War boardgame handles it. However, some would argue that a colonial squadron should be limited to the local tech level, per TCS. Me, I have real problems thinking that the Imperials are going to build the far future equivalent of nuclear attack submarines and missile cruisers while the colonials they look to for support are left to build the far future equivalent of steam-powered ironclads. If the Imperials are depending on the Colonials to carry any weight, they'll make sure the ships used by the colonials are at least a credible threat in battle.
Of course, those without Imperial ties are left to their own devices. IMTU, the Swordies fly TL13 ships bought from the Zho and often serviced by Zho techs, simply because with only their own tech they wouldn't have any value as a military partner and wouldn't be the slightest threat to the Imperium.
"Would there be a mix of jump capabilities?" Yes, but not generally in the same fleet. High-jump ships are wonderfully "fast," able to outmaneuver slower fleets to respond to changing circumstances, to threaten the slower fleet's line of supply, or to strike fast and deep into enemy territory. They pay a steep price for that in firepower. Slower fleets can be impressively armed and armored - but they trudge along like cosmic turtles. In essence, you've got your slow "heavies" and your quick but less potent "lights" complementing each other and doing different missions on the strategic map.
"How many escorts and logistic ships would be included?" However many are needed - which in turn depends on your logistic rules, which there aren't many of out there from which to draw conclusions. That might be one where you make up whatever sounds reasonable, because there's not much to measure against. As to escorts, a fleet needs eyes forward, aft, and around - those are the escorts. How many are needed depends on what range their sensors have, what ranges weapons have, how tightly the core fleet units form themselves, and so forth, so enough to make sure the enemy's approach is detected in time for the fleet to prepare a response - and to make sure some light skirmisher doesn't penetrate close enough to obtain intelligence you don't want him to have.
If you're using High Guard, the other consideration is how long can they survive on the line covering the retreat of the big boys? You want your escorts to be able to survive a terrible beating for a brief interval when you want your important ships to try to make a break for it, so you need enough escorts to do that - and of course they need to be very agile and small enough to be a hard target.
"Would each fleet have some form of a fighter/carrier capable ship included?" Yes, but not a large one, and for some of the same reasons you need escorts. Fighters are of no use in battle, but in a system they can scout far afield of the fleet to see what's around, they are cheap enough and numerous enough to scatter if they meet substantial opposition, and they're cheap enough that no one but the next of kin will weep if they get destroyed gathering your intelligence. However useless they are in space battle, they are very, very useful in commerce raiding and ground attack - in the latter role, they're basically as good as any tank and a great deal faster. So, a fleet will most likely have a dedicated carrier to serve that role - most likely a variable number depending on what the fleet's current mission is. I'd expect the carriers to be attached and detached in an as-needed capacity, like other specialized support ships.
"How many of each kind of ship would there be in the subsector fleet?" Yeesh! Depends on what the need is. Too many variables for an easy answer.
"Would the fleets in contested subsectors be heavier and lighter in safer subsectors?" Maybe, maybe not. Canon says the Imperium moved away from the "crust" concept of heavy forces up close to the front because the opponent was still able to concentrate force and punch through the crust. Instead, they concentrated forces a ways back, close enough to come forward and get in front of the enemy's main axes of advance once they were known. Don't know how non-Imperial governments handle it.
"Would the make up differ greatly from year 900 to 1100?" Probably not for the Imperium, not for the type of ships composing the fleet. Lightning class cruisers, a TL14 design, were coming out in 993. Some of them were converted to frontier cruisers with a TL15 Factor-N meson spinal starting in 1080, so somewhere in there is the move from TL14 to TL15. Not such a great difference between TL14 and TL15 that it would alter the balance of the fleet - power plant improvement means more room for armor and weapons, but the balance doesn't change: mesons are still the wicked killer encouraging you to keep size down, fighters are still pretty much useless. Other places, I don't know.
