My ISP (Ziply) allows for 2 DHCP leases at a time at the ONT, this is extremely useful when swapping out a router. I ran two routers for a while without issues. They do this on purpose but they still control the bandwidth allocation at the ONT and I don’t get twice the speed by using 2 routers.
The internet itself is shared. If that’s the case, put a router in and you are protected as with any ISP.
The ONT gives out an Ethernet signal. If the Google WiFi gets the IP address you are good. The one thing that the ISP can say is that you are not supported if you have an issue because it’s not connected to their router. If that’s the case swap out routers when you have issues.
I’ve been running Google WiFi straight to my ONT for years.
If you have the original Google WiFi pucks, I have recently flashed them with OpenWrt and it has been great.
Took a bit of time to fully set them up properly, got locked out and had to reset them multiple times. But am super happy with what I have now. Running multiple VLANs, a real firewall and the WiFi seems much better.