@Pemtium 

As long as you run Deco mesh in AP (Access Point) mode, this setup will work. But, why M9/M5 combination?

M9 has dedicated backhaul, but that only works on two M9 connected to each other wirelessly. Without that, M9 performance is only marginally better than M5, you might not notice the difference.

If you want cost efficient setup, go all M5.

If you want fastest WiFi possible in your environment, go all M9.

If you want future proof your home network with WiFi 6 support, consider Deco X-series (X20 or X60) instead of M-series (M9 and M5). 

@Alexandre. , Thanks

No Decision on Device model Yet.

Just wondering. i really appreciate your recommendation

so should I go all six X20 or all X60?

Also something i get confused about your reply 

        "As long as you run Deco mesh in AP (Access Point) mode, this setup will work. " (this is a Setup on The Device, right?)

Any Consideration from all six X20 or X60 (what's the main difference? (if you have time to answer/reply)

(3 will be Wired)

What are the Dimension (base width and height)?

Also I should try 3 first and see how it goes.

Thanks in advance. Again