What do you mean "Speaker locations you were given"? Who wired this and/or designed this?

For professionals, there are many things to consider in looking at a system (just concentrating on audio here):
I have been hired to design, install, and calibrate a proper audio system. That means "I" am the one that dictates what equipment is used, where the equipment goes, and how it hooks up. If someone else (designer, customer, or family friend) is doing any of this, then they are not looking to hire a real professional are they.
There are times that compromises have to be made in the design/setup of the system. "I" am the one that determines what those compromises are and how it is implemented. If someone else is dictating how this is done, again they are not really looking for a professional (and care how it turns out) and they don't need me.
All this above is simply stating that if a customer doesn't want to do things at least semi correct - it is up to you to determine if you want to do the job or not. It is YOUR reputation on the line and name on the system.

A couple of points on this system specifically:
Front speaker array has to tonally match! This is a non-negotiable item. If you have Leon for a center, you need to use matching Leon speakers for the left & right. This would mean the Leon Vault in-walls.
But you said the left & right are 8ft up the wall? How high is the center channel? If the center channel is going to be 4ft off the ground (resulting in a 4ft difference center to left & right) - that is a HUGE NO-NO!!! Do NOT do that BS! If the left & right can not be installed within 10-15% of the center channels height 'or' you can not mount the sound bar above the TV to get near this, then do an LCR sound bar on the TV.
The rear surrounds are not as important to tonally match (as long as they match each other). So those could be another brand even if need be.