Yes, it is possible to link a Google WiFi network to your main Wi-Fi connection.
Yes, it is possible to link a Google WiFi network to your main Wi-Fi connection.
I aim to link wired gadgets connected to my ISP router and integrate them with Google Wi-Fi. For instance, if I’m streaming an iPad over Wi-Fi to a TV connected to my ISP box, or can Google Wi-Fi act as the central hub while routing the ISP router for extra Ethernet capacity? Is this feasible?
Seems you're operating your network with double NAT, which isn't the best setup. Two routers are handling separate networks. When you chose Google WIFI, it's better to replace your ISP router entirely—it functions as a router itself. You have several choices: eliminate the ISP router, use it in bridge mode as a switch, and disable its wireless radios. Google WIFI should also run in bridge mode to act as your wireless access point.
Also mentioned by @rickeo, using Google Mesh in AP/bridge mode will remove mesh features. To keep mesh functionality, you must switch your ISP gateway to bridge mode.