What I understand is that the powerline devices can be connected via spur to spur. But you need to make sure they are in the same circuit or ammeter. And devices with built-in surge protector will reduce the transferring speed.

The devices must belong to the same electrical (not ip) network to implement one powerline network. This means that the plugs you are using must be connected to the same electrical "phase" or "segment" if your building has 3-phase power supply. If your building only has 1-phase, then all you have to do is avoid any multiplugs with surge protection, filters etc., although I have read in this site that in case of interference these might help. I would propose trial and error to find out. Always first try to have only the PLC on the power socket. Also try to avoid plugs that have airconditions or other inductive loads on the same segment.