Google WiFi and Airport Time Capsule

1. If I extend my Google WiFi (by mesh or by wire), can I connect Time Capsule on secondary Google WiFi?

Yes, assuming that the Time Capsule will connect to the Google WiFi access point using a wired Ethernet cable connection.

2. Can I use the LAN port on Time Capsule as hub for wired stuffs (e.g. Hue Bridge?). Reason is I could hide them at the ceiling of corridor instead of putting all wired stuffs in Living Room.

I'm not sure exactly what you are asking, but the three LAN <--> Ethernet ports on the Time Capsule will all be enabled, so you could connect an Ethernet device to each LAN <--> port, or even connect an Ethernet switch if you need more than three LAN <--> connections.

It may or may not be of interest to you, but Apple does not officially support Time Machine backups to a Time Capsule unless you are using the Time Capsule wireless. That does not mean that things won't work if you are using another wireless network.....it means that Apple will not offer assistance if you have any problems with your setup in the future.

So as long as things are working OK, you might not really care that Apple does not officially support what you are doing.

The WAN "O" port on the Time Capsule will be used to connect to one of the Google WiFi access points, but the three LAN <--> ports will be available for wired devices.

If you need more Ethernet ports, you can add a 5-port, 8-port, etc Ethernet switch to one of the LAN <--> ports on the AirPort.