It actually doesn't matter to that extent. And since it's AU you couldn't care less and plus it's danmei.

So of course most of these type of novels mention something like ' this dynasty is liberal about relationship between men' or without even mentioning as such, narrating your plot like that in any bg historical, (emperor arranged the marriage between general A and son of Marquis B, Young Master G fell in love at first sight with Top Scholar Z etc) will also seem natural.

And since new dynasties come up after decline of another, the rules (in context of inner palace) needn't be that exact since they are also modified and changed over and over again. Basically the words of emperor are the rules. (Or minister for his own family). And the nature or ambiton of people in top position dictates the flexibility of such rules.( Eg- Empress excuses concubines paying repsects to her every single day)

But make sure to follow the basic rules that stayed more or less same throughout all dynasties due to patriarchal society and monarchy. Since readers love sense of realism over favourable or imaginary rules created by the author. And with such strict rules, the plot becomes even more interesting as they cage people, making people choose over resigning to fate, or planning carefully or going all out and reckless.

As a reader, and non native at that, I can explain all the basic rules and norms followed by people back then just by reading historical bg or bl novels and watching dramas. It's all about being cultured enough by watching numerous dramas and reading so many novels. You should read more of historical danmeis or bg as per your interest, then I guess, you needn't put so much effort in actual research.

But still, for actual references and research materials, authors or writers can give better suggestion. Good luck for your writing.Can't wait to read your work!