How to decide on what citation format to use when writing on legal history?
If you are hoping to submit the book to certain publishers you could check what guidelines they have for authors. For example, here is a link to the Oxford University Press

I would not recommend to use the Chicago style for the history sections and the law citations for the legal section. Using a consistent citation style across your manuscript would normally be expected, and again some publishers may expect you to use a specific style.

If you are using a citation style used by your publisher I don't see why it should impact on if your work would be cited or not. The content of your work and if other academics use it should be what would gets it cited.