500Mbps powerline adapter + 200Mbps powerline adapter, the owerline rate is 200Mbps. this is the powerline rate which is the transmitting rate between the two powerline adapters.
the bandwidth between the router and PC depends on the bandwidth provided by the ISP.

Thanks Joyce for your quick reply.

Unfortunately, it does not completely answer my question. I'll try to eloborate a bit. The situation will be as followes

Adapter 1 = 500 Mbps, connected to router
Adapter 2 = 500 Mbps, connected to MediaPC
Adapter 3 = 200 Mbps, connected to NewPC

All three adapters will be in the same powerline-network.
I understand that the Powerline rate between Adapter1 and Adapter3 will be 200 Mbps. But what will be the powerline rate between adapter 1 and adapter 2 (so during streaming media files from the media PC to the router)?

Will it be:
A - 500 Mbps because both adapters are 500 Mbps, even though there is a 200 Mbps adapter somewhere else in the powerline-network

OR
B - because there is a 200 Mbps adapter somewhere in the powerline-network, all connections within the network will drop to 200 Mbps

B
the whole powerline network rate will be 200Mbps

Use the powerline utility's - when the units are connected you should see the connectivity speed. I had a 200MB and a 500MB unit linked together - due to the distance my speed was showing at 70MBs between the units

the whole network will drop to 200 because the adator only goes upto 200Mbps and your 500Mbps adators cannot make your 200 go to 500 so they will drop to the lowest adaptor size.