@Anthony2816 

I would first look at it of if I want all the features of the Deco P9 such as parental controls, qos, etc. If not, and your main purpose of the Deco is to provide Wi-Fi coverage, you can do the first setup and have the Deco in AP Mode.

The second setup is more ideal if you want the Deco to be the brain of the network. Other then having it plugged into the gateway, you would want the gateway bridged so it just passes the internet connection to the router. You really do not need to, lots of people have a router connected to another router. If anything, in the future if you notice any speed issues that would be one of the things to try. With respect to the other satallite units, you would wire those into the Ethernet switch.

@Tony 

Thanks, Tony.

The Comcast gateway doesn't allow for bridge mode, but I'm going to replace it with a modem in a few days, so as I understand it I'd plug the modem into one of the first P9's ethernet ports, and plug my existing ethernet network into the second P9 port.

Then, if I decide the P9's limited router functions are too...limited, I could place a separate router (with its wifi disabled) between the modem and first P9, and plug my ethernet network into the router, and put the P9 into AP mode?

@Anthony2816 

Exactly