Through which IP address will my server be communicating if I use a Floating IP address?
In case you use a Floating IP address, the server will be communicating with the router IP address as its source address. You can verify this by issuing the following command:
dig +short myip.opendns.com @resolver1.opendns.com
The response will show you the IP address from which your server is communicating with.
You however are allowed to set the floating IP as default gateway instead of the router IP. The instance will then use the floating IP as source address.