There is, as you suggested many old "rules of thumb" for such applicaiton. Another is "2 and 3...3 and 4" which translates into the distance in inches between joist pockets on the top of a Summer beam and the distance from of the pocket on the bottom...min and max.
I'm found of soffit and/or tusk tenon for such applications, not dovetails...even if the tenon has to be "free" and slid in.
Your timber sizes seem more than adequate if I'm understanding the overall design without seeing a CAD or drawing, so no issue there.
Your reduction of the joist (aka scoop cutting) seems acceptable but not necessary at all, and would only be dependant on aesthetic you are trying to achieve. A fully house joist appears achievable if it is desirable...
As to the "smallest pocket possible" that motivation for strength maintenance is not really a concern it would seem? As to cutting the pocket, a "v" gouge (aka corner chisel) and a #5 in or out cannel gouge would make short work of this task in either green or dry oak without much effort. Then simple cleanup with slick or paring chisels...Of course any applied power tools and/or saws can be employed according to skill set, tooling and applied knowledge to effectively use them of course is also part of the tooling should you have these?
Hope that was of some use...
Last edited by Jay White Cloud; 04/12/20 09:16 PM.