@VargU 

I can provide answers to questions 1 and 2.

>Question 1:
>Can all units of the DECO series be combined with each other? (e.g. Deco XE75 6E + Deco X50 + Deco X20).

Yes, they can.

I would recommend not to mix Deco devices that support different WiFi protocols. XE75 is WiFi6E, the rest on your list are WiFi6. If I were tasked to use such mix, instead of XE75 I'd rather use X68 (tri-band), X55 (three Ethernet ports) or especially if this is for the Main Deco - X90 (top of the line Deco in WiFi6 X-series).

While Deco XE75 6E + Deco X50 + Deco X20 should work together, you might encounter issues with devices roaming between WiFi 6E and WiFi 6 nodes. 

>Question 2:
>Can all devices of the DECO series be used as access points behind a TP Link router?

Yes, they can. See How to set up Deco to work in Access Point mode and What’s the difference between Access Point mode and Router mode on the Deco?

>Additional question, if yes:
>Can the LAN ports on the DECO devices still be fully used when they are used as APs and will the devices then also be displayed via the router?

LAN ports can be used to wire devices to your home network, without issues or limitations. You can even attach regular gigabit networking switch to Deco (either Main or any Satellite), and connect multiple devices to that switch.

When Deco runs in AP mode, devices will be displayed in the router device management web page.

Hello @Alexandre. 

Thank you for your support.
The links were also very helpful and we had not yet considered them. Thank you.

On question 1:
Very good point and advice. Will be taken on board. Thank you.

On question 2:
Great and thank you for the further information.
If I understand correctly, we still need 2 apps or web UI for the complete setup and further administration. (which would not be a problem)

Another idea - and this would also solve question 3:
Is it possible to set up 2 networks via one internet access?
E.g. 1 x Omada + Access Points and 1 x Deco alone.
If possible, how would one set up such a constellation?

Thank you for your time and efforts

  @VargU 

I really can't answer any questions involving Omada. 

One thing I think I must to clarify:

>Can all devices of the DECO series be used as access points behind a TP Link router?

Deco will not extend TP-Link router WiFi. Deco always runs its own WiFi. It is recommended to turn off router WiFi after Deco is configured, or you'll end with two separate independent WiFi networks.

In Deco Access Point configuration, TP-Link router will keep managing network (IP address allocation, DHCP lease time, DNS servers) while Deco will provide WiFi coverage.

Now, to your comment:

>If I understand correctly, we still need 2 apps or web UI for the complete setup and further administration.

This is correct. You will be managing TP-Link router same way you do now. You will also manage Deco mesh through Deco app. This is important, too: you can only manage Deco mesh through app, there is no WebUI to configure, setup and manage Deco mesh. You must have smartphone (or non-Windows tablet, perhaps, not sure about that) for managing Deco mesh.

  @Alexandre. 

No problem, but thanks for the further explanations and help. That helps quite a bit.

Let's wait and see if we can shed some light on the subject of Omada :)

Have a good time