@DickPowellUK 

Deco X50 will be compatible with Deco X55, except maybe missing some of the features if firmware of X50 is behind. Firmware versions will depend on X50 hardware version.

I checked TP-Link UK web page, it appears the best for X50 would be hardware V1, followed by V3, and absolute worst is V2 - very behind in firmware releases. My suggestion would be when you get X50 set from Amazon, don't open it - you should see hardware version on the label of the box. If it is not the one you need, especially if it is V2 - just send it back unpacked. Will save you a lot of time.

See: How to find the hardware version on a TP-Link device

Also, take a look at this document: Understand the Differences Among Deco Firmware Platforms

If you don't use any of the features listed there, you can add any X50s to your Deco mesh without any concerns.

  @Alexandre. 

Many thanks for your reply Alexandre!

To be frank, I don't understand why they dont sell the X55 in twin packs or solos. Very annoying.

I am not an expert!! I looked at the pack for X55s and its says Ver:1. Though I don't know if that refers to the Firmware. However, the App for new Deco system says there is a firmware upgrade to be downloaded: Version 1.4.1, Build 20231122 Rel. 40485. After what you said, i am a little fearful of upgrading the Firmware now!!

I need to do a bit more price scouting - buying a solo X50 is almost as expensive as a duo pack. And then maybe, i should just buy another three and use them around the house (it's a 7 bedroom house).

Best

D

  @DickPowellUK 

You should definitely upgrade firmware: new version may bring more features and better stability.

When you mix different Deco models in one Deco mesh, they will run together, but the feature set available will be from what these models all support. For example, you may find it necessary to configure how Satellite Deco connect to each other or to Main Deco. For deployment of your size (5-6 units) that could be necessary feature. It is supported by X55s, but if you add to them Deco model which does not support this feature - it'll disappear from Deco app. 

Deco X50 V2 does not support it, as far as I can tell.

For 7 bedroom house, I would suggest just buy another set of three X55s. You can't have too many WiFi mesh units. For comparison, I have 3 bedroom house with 4 Deco units. Since I deployed Deco mesh I have not had WiFi related complains from members of my household. Before Deco, it were a different story, no matter what I tried to get good WiFi coverage through my house.