Are you still working on this?
From my experience (with R2017b), this kind of control cannot be done with only Entity Queues. The setting to have queueing by priority stops at choosing the policy and that's it.
To have the behaviour you expect, I imagine you might need a combination of Entity Queue and Entity Gate (and somehow put a limit) or you need to manipulate the value of the entity priority with Event actions (so that after 5 high priority messages, the next high priority messages will have a lower priority while 1 low priority message will have a high priority, something like that) either in the Entity Queue or in some kind of pseudo Entity Server (service time=0) just before the queue.
I am not sure myself if it is possible and I have not tested anything, but I may try if I have the time.
