Here's my take on the Imperial Navy. For sources I use primarily the "Fifth Frontier War" boardgame and the Batron description in the "Spinward Marches Campaign". This defines the Imperial assets available for the war (only a portion of the total Imperial assets).
Analysing the counters (assuming I haven't lost any over the years) …
Batrons Number: 32 of 11 different types Defense Factors: 147 (average 4.6 per squadron) Crurons Number: 26 of 7 different types Defense Factors: 163 (average 6.3 per squadron) Assaultrons Number: 8 of all same type Defense Factors: 48 (average 6 per squadron) Tankrons Number: 2 of all same type Defense Factors: 8 (average 4 per squadron) Scoutrons Number: 8 of 4 different types Defense Factors: 38 (average 4.8 per squadron) Colonial Batrons Number: 10 of 4 different types Defense Factors: 67 (average 6.7 per squadron) Colonial Crurons Number: 19 of 9 different types Defense Factors: 107 (average 5.6 per squadron) Colonial Assaultrons Number: 1 Defense Factors: 4 (average 4 per squadron)
From this I make one deduction - that the Imperial fleet is very un-homogeneous (in contradiction to FSotSI), and one assumption - that there is one capital class ship per defense factor. In other words there are (within the Spinward Marches at that time) eleven different classes of Imperial battleship/battlerider (plus another four classes used by the Imperium's colonial forces). There are also seven different classes of Imperial cruiser (plus another nine classes used by the Imperium's colonial forces). It would seem that the smaller the ship the more standardised it is likely to be (for Imperial forces), but colonial forces use more Crurons.
Additionally, there are no destroyer squadrons. So they (and other escort vessels) are subsumed into the squadron types listed above. Thus each defense factor represents a capital class ship and its escort wing. (Of course, a destroyer squadron could be labeled a "Scoutron", which would explain what a Scoutron capital class ship is.) In a Battlerider squadron each battlerider is counted but the battletender is only counted if it is combat capable (most are not, but the Lurenti class for example is).
In war time the activities of a number of Batrons and Crurons are coordinated by a Fleet Admiral, and as such are considered a Fleet. Assaultrons and Tankrons may be added depending on mission profile. So called Scoutrons are primarily for inter-fleet use. (For example, IMTU the Imperial 214th Fleet at Glisten is composed of 2 Batrons, 3 Crurons, and a Colonial Cruron.)
In peace time Battleship squadrons are broken up into individual Dreadnaughts plus support ships and escorts (like a current day US carrier group) and are deployed to potential trouble spots. Battlerider squadrons tend to stay in Depots or are out on training missions. Cruiser squadrons are broken up into pairs of Cruisers plus support ships and escorts ... called CruDivs (Cruiser Division), that patrol Imperial space. Scoutrons are disolved and deployed as individual vessels. Assaultrons and Tankrons stay in Depots and at strategic points throughout Imperial space.
The capital class ships in Imperial regular squadrons are primarily TL15, with some TL14 ships awaiting retirement. (Retirement means either mothballing or transfer to the colonial forces.) The capital class ships in Imperial colonial squadrons are either transfered ex-regular ships or localy produced ships at inferior TL (13 or 14).
To summarise: The Imperial Navy term "squadron" (when used as Batron, Cruron, etc) does not just mean a number of identical ships, it refers to a mini fleet. During the 5FW the Imperium Navy had 404 regular capital class ships plus 178 colonial capital class ships in the theater of operations (plus all the lesser ships) ... 582 capital class ships Imperial total in theater.
The exact organisation of planetary defense forces varies from system to system. Some have a single integrated military, others have separate space navy, COACC, air force, wet navy, and army. For the purposes of Imperial military planning these organisational distinctions are largely ignored - from the Imperial perspective the planetary navy is classified as the system's SDBs, orbital defense platforms, interplanetary patrol ships, asteroid monitors, and (when they can afford it) a system might even have its own naval squadrons.
While the Imperial Navy is concerned with protecting the interests of the Imperium as a whole, there is no guarentee of local protection. To that end the Imperium authorises subsectors to raise their own navies - individual systems contribute to a subsector-wide military alliance. In peace time a subsector navy is under the control of the subsector government, in war time control is passed to the Imperial Navy ... but there remains an understanding that such forces are for local defense.
Planetary navy squadrons and subsector navy squadrons are collectively referred to as "colonial squadrons". Colonial squadrons represent a local reserve force. Used Imperial ships are available at a discount, and there is an officer exchange program with the Imperial Navy.