@wizzard1 

Yes, there could be interference, but it is hard to tell how bad it'll be until you try. As long as there is sufficient air flow for Satellite XE75 not to overheat, it will be safe to try. Whatever electrical/WiFi interference there will be, it would not harm XE75 hardware.

Make sure it is not completely sealed air tight space, place XE75 Satellite there, check in Deco app it has got good level of Signal to the rest of the mesh and run few speed tests with devices connected to that Satellite Deco.

  @Alexandre. 

Thanks for the reply. The intension is to place the Deco under the stairs (not air tight and plenty of room as well). My Alarm security guy suggested that I should connect the Alarm box internet cable to the Deco for better performance. Currently the Alarm box inter net cable is connected to TP-Link Powerline 600 socket and the other end to TP-Link Powerline which connects to Deco Base Router ethernet at the back. 

Question is, if I were to use the satallite Deco ehernet to Alarm ethernet, will it be an Ethernet connection or a wifi connection (in other words will I need to provide a SSID  and a password to Alarm?

  @wizzard1 

As far as Alarm device concerned, if you connect it with Ethernet cable to Satellite Deco it will use it as Ethernet connection.