@larsvi,

The two are very similar and will provide a similar experience - with the XE70 pro being the 'successor' to the XE75 pro. You shouldn't have any problem using these in combination, the only advantage that could potentially exist is that with one model, your entire network would upgrade to a firmware at one time - so if there was a feature introduced in the future (none that I have seen) that requires your entire network to be up to date, it would theoretically happen more quickly.

Overall, you have nothing to worry about. If needed you may consider event using your extra node to provide wired connections to a corner of your home where the signal is weaker, or close to your backyard to try and forward a stronger signal there.

As for Matter, I am not sure that these have compatabilty as that comes with a 1.3.0 firmware update. But I should mention that this is only for controlling your devices from the Deco App, you can always use Matter with another controller such as a Google Home, Alexa, or Apple TV or just the Tapo/Kasa Apps. Plus, your decos can still handle and forward the Matter Protocols traffic correctly.