I am only guessing here see i get unlimited network adapters through proxmox I at this time don’t let my web server run on the host machine only for security purposes however… From my experience in the data center where i am located the one machine is assigned a public ip and I have 12 other ips i can appoint by mac address… I have had 4 internal networks to separate out traffic from some of my other servers and it worked well with a few snags here and their… I do know that you need to label each network card on whether its local or internet traffic…
Best advice i can give is to try and test what your doing on an old pc just to be sure before putting it in production environment…
I actually run two nethserver just to ensure my uptime… One at my home and the other at the data center.
Here is an article I found from awhile back and think it could possibly relate in some way on what your doing…
http://docs.nethserver.org/projects/nethserver-devel/en/latest/gateway.html