Yes thats right i believe that you will need the same brand of homeplug, for them to all work.
They need to go on the same circuit though.
They don't, they just tend to work better if they are.
How do they talk to each other then?
All of the circuits in a house are interconnected to some extent. General rule of thumb is that any circuits connected to the same consumer unit will be okay.
You do occasionally hear about accidental connections between different buildings, so it's always worth setting your own encryption keys rather than relying on the factory defaults.

Just bought a pair of TP-Links. Ridiculously easy to set up (compared to my previous experience of anything to do with networking anyway). My main PC has 31.3 Mbps reported by speedtest, and I'm getting 10.1Mbps through the powerline adaptors. Its quite an old house however and its virtually the full length of the house apart. Is that sort of performance par for the course? I bought it to stream Premier Sports through my TV and its not working out very well tbh
