@Miguell Good day, 

In theory, all the TP-Link powerline devices on sale can work together. In the real network environment, it would be better if we use the powerline devices with same powerline data speed in the same network. If you have 7 series powerline adapters, it is recommended to buy 7 series powerline units as the extenders to get better performance. 

If you 'd like to use powerline devices with different data speeds together. Here are some recommendations:

@Miguell Hi,

Thank you for your reply~ 

Indeed, 8 series has higher spec compared with 7, normally when different power line devices are in the same powerline,  we recommended same series power line adapters to work together for better  communication and best performance. 

If you'd like to test, you could unplug old powerline and test 8 series performance only and see if signal is getting better. 

Btw, may I know do you have multiple rooms which have Wi-Fi dead spots, as simply adding more powerline devices does not always boost its performance for a certain spot which you need better Internet connection. As you mentioned when use the old adapters, there is already some issue when playing games or in VC calls.

Extender powerline unit is paired and communicate with the main adapter, if more adapters are added , more extender will share the total bandwidth from router/ main adapter.