You mentioned during the co-existence section a the beginning you will require multiple gateways for a single vNET to host S2S and ER in a Co-existence??? is that correct? i thought you cant have more then one gateway per vNET? Co existence in my understanding would use different interface on the gateway VM??
Yes you would a gateway type VPN and a gateway type ExpressRoute. Both would be created in a GatewaySubnet and they can coexist. You can only have one Gateway of each type in a Vnet. So once you've created those two Gateways you've hit the maximum. You can create up to 4 ER connections through one ER GW and depending on the VPN Gateway performance (and cost) you choose, you can create up to 30 VPN connections.