@E2XD 

1. Make sure you are buying XE75 or XE75Pro if you want 6GHz network. Deco X75 is WiFi6 only, not 6E.

Deco X75 specs: https://www.tp-link.com/en/home-networking/deco/deco-x75/#specifications

Deco XE75: https://www.tp-link.com/en/home-networking/deco/deco-xe75/#specifications

Check WiFi Speeds section under the specs.

2. In my region, I can't buy single XE75, they only come in pairs or in sets of three. If it is the same where you live, you may have to go with more expensive XE75 Pro, they are sold in single units according to this: https://www.tp-link.com/us/deco-mesh-wifi/product-family/deco-xe75-pro/

3. Deco XE75 and X50s will work together in one Deco mesh just fine. Mobile device that runs on WiFi5 or WiFi6 won't have any issues with that mesh. Mobile device that supports 6E and 6GHz should use such Deco mesh without issues, but may not roam as well and as fast as you would expect it to. Device will have to decide how long to stay on faster 6E and when to move to slower WiFi6/5 but with stronger signal, and vice versa.

This is mobile device specific, can't tell if that will be a problem with devices you have until you try.

4. If your Internet speed is gigabit or less, another option would be to go for the top of the line WiFi6 Deco instead, which will be Deco X90 or X95. This will eliminate potential roaming issues and hopefully these devices have enough CPU power and antennas to support all mobile devices connected to them.

It looks as X90 and X95 are not sold as single units either.

  @Alexandre. 

Hi Thank you very much for your answer

(and the clarifying the confusion on my typo X75 instead XE75). :-)

It was very useful and easy to understand.

Best regards