Yes, it is very tight as Stardustone has already written. I have been using this device for a few years. It is plugged directly into the 3-socket outlet, but before that, an extension cable is plugged into a free outlet, which just about works.

No, it definitely doesn't fit, because the housings are unfortunately in each other's way and part of the triple socket is covered in such a way that there is no longer enough space to connect more devices. There's not much you can do about it, because the PowerLine devices should go directly into the socket, so I wouldn't extend it.

But: I generally advise against buying a PowerLine with 500Mbit/s, because that's money down the drain. Today there are solutions that can do more than double that. Because with PowerLine it is very important that you start with high speed, because otherwise you won't get much out of it at the other end. That's why I recommend at least PowerLine AV1200 products, such as

Devolo Magic 1 LAN Starter Kit 1-1-2 (1200 Mbit/s)

At least you still have a socket on the PowerLine device that you can continue to use.