CommsGuy, this seems a hard decision to make, because you're not sure and that is understandable. The only way to be sure is to decided to do the hard work and setup your beam in the vertical with and without the Imax,
First, just to be sure you have good control over the beam.
Then add the Imax above the vertical beam.
Then, if the combination did not work as you wished or was not good performance wise, then set the beam to horizontal with the Imax above.
Then you could decide if you need another tower...which would be my best choice of all, because that it the way I preferred to do it on top of a 17' foot high roof deck on my house, even though the combination of horizontal beam and vertical GP seemed to work very good together.
Personally, I never gave a vertical beam a second thought.
This is not to say that I had no luck or bad luck with a vertical ground plane above my horizontal 4 element M104C home-brew. And, these setups of mine included an A99 and a Starduster on a 10' foot high mast where the GP radials almost touched some of the elements on the beam, due to they're being slanted down at a steep angle.
As you say the mix of horizontal and vertical is about as good as you're going to get with these antenna working close together, and that is due to the big difference in polarity, which almost makes these 2 antenna invisible to each other.
Good luck and keep us posted.