@pogologo 

I have similar Deco mesh setup: I bought 2-pack M9 Plus, they communicate through wireless backhaul. I configured them in AP mode and was very happy with their performance, but decided I need one more node for a specific place in my house, for ideal coverage. A week later, I've added M5 there. I wired M5 with Ethernet cable.

My Deco mesh is now Main Node M9 Plus, Satellite Node M9 Plus and another Satellite Node M5.

M5 works just fine in that mesh. I disabled 2.4Ghz network, my house is covered with 5GHz WiFi. I have not found drop of performance in my mesh after I've added M5. I have 300Mbit Internet connection, and I was getting 300Mbit download speed on M9 Plus before and after I've added M5. I am also getting 300Mbit download speed on M5. 

I tested Deco mesh capabilities by streaming Youtube video on smartphone while it was connected to M5, then I started to move towards my Main Deco M9, then went to where Satellite M9 is. The video streaming was not interrupted or buffered while I walked around. I checked that at the end of my walk smartphone was connected to Satellite M9 - meaning smooth transition between all three Decos, including M5.

Deco app handles different types of nodes on same network well. It reports proper firmware version for each node. Just recently new firmware was released for M5, Deco app offered upgrade and it went without any issues. I now have M5 on new firmware and M9 on whatever they had before.

All in all, I am more than happy with TP-Link Deco mesh design, where it is possible to use different types of Deco hardware in one mesh system. 

@Alexandre. 

Thank you. Really appreciate you taking the time to reply.