Biggest problem that I have is that it is said specifically that the ComStar Expeditionary Force consisted of seventeen Divisions and fourteen Independent Regiments.
Sure, you could cut those numbers for the TRAS, but I wouldn't. After all, none of those should be anywhere near full-strength (pre-Coup). Perhaps they should be (after all, if I was Commanding General I would have merged Divisions and Regiments to bring those that would remain on the rosters back to full-strength, even if I had to fold the colors of a (fairly large) number of storied and distinguished units), but I kind of doubt that they were.
And recruiting and rebuilding the SLDF was not a top priority (or any kind of priority!) for the Star League Council in the aftermath of the Liberation of Terra!
My own thoughts? Use all seventeen Divisions and fourteen Regiments, but have them around 54-55% of their nominal "book" strength. That would give you the numbers you want, but with keeping the units that must have comprised the CSEF.
Now, you could merge some Divisions and Regiments in Shattered Dawn, cutting those numbers down to where you are aiming for and it would still account for all of them. Just say that the (for example) the 328th and 359th BattleMech Divisions were merged with the 151st Royal, 213th, and 295th Division bringing the 151st and 213th up to 100% TO&E strength and the 295th to 75%. Takes less than a paragraph and neatly explains where the other units went (of course, choose your own survivors from the list). You can even steal man, material, and equipment from a Mech Infantry Division to bring that third BMD back up to 100%.
That way you could get nine Divisions at full strength (or nearly full strength) and still stay true with what has been published.