I will need correcting by MB Studio on what I will say here as I am guessing on some solution to artist separation which I could be wrong.
I am not sure how the master setup/songs rule takes priority. I will assume setup/songs is overtaken by the individual song properties/rotation if setup/songs is set to zero as song properties/rotation default is zero as I have 1 hour separation in setup/songs and I do not play back to back same artist within an hour with songs/rotation set at zero.
If this is the case
In setup/songs you would have to disable (set to 0) artist separation.
This means you will now have to use song properties/rotation tab artist separation rule for all songs in your library which is work for you to do as they are all set at zero.
You would search the Genre you want no separation rule for and you would set it for 0 hours for each song of that Genre if you changed the default.
All other Genres would have to have their rules set as you set the master song separation (setup/songs) to zero, so here is the work for you in all the other Genres in setting separation time if you want the artists to be separated by some time period.
Now if the Genre with no artist separation is only played in a certain hour you could set those songs in that Genre be allowed to be played a certain hour/hours and have zero artist seperation.
Side Note: if it is being streamed it would be a violation of DMCA rules for playing more than 2 plays in a row of the same artist (I believe)
Hopefully this would work, but I don't always understand the questions posted.