Imperial standards are discussed at length in T4 - see Central Supply Catalogue and Emperor's Arsenal. They also get a mention in T5/AotI.
The Sylean Federation was already TL12 when it became the Third Imperium, the ACR was old tech even then, so standardising a TL10 design for mass production in TL12+ makers is not a problem, and enforcing standards across the Imperium is sort of the only reason for an Imperium (one of many only reasons to paraphrase Arthur Isaacs first rules of warfare).
Finally the Imperium of 1105 long since ceased expanding, the authors have been quoted as saying it was a mistake to have the Imperium so surrounded, and in the original Spinward Marches it is described as "under strong pressure from its neighboring interstellar governments, and does not have the strength nor the power which it once had. The seeds of the rebellion and Imperial collapse were there from the start, more obviously than the great Aslan secret too.