As mentioned, 7b2 has a lot of extra’s. And upgrading from 7b2 to the full version will be like do a regular update of the system.
I understood upgrading from 6.8 to 7 will be available too through the web admin page. I think @davidep will be able to tell you all about this.
So if I understand correctly, you have 2 floors with about 10 clients each. Ultimately, they could be in the same subnet because it is all 1 company and everybody should be able to use all services. But since you have a separate switch on the other floor, they are physically separated and getting another subnet.
You could just put the cable from the switch on the other floor straight into the switch on the main floor. This would connect both floors and you have the same subnet on both floors. I don’t know what type of switches you use, but keep in mind that all the users of the other floor will share the bandwidth of the connecting port. Some switches have SFP ports with higher speeds or the option to bind multiple ports for more bandwidth.
It would avoid the hassle of creating routes from the 2 internal subnets. It also will make it possible to use roaming for wifi users if your accesspoints support that. It would also mean that you only need 1 interface for your internal network on your server.
Multi Wan is very well explained in the documentation: http://docs.nethserver.org/en/latest/firewall.html#multi-wan
I strongly suggest to read the Networking documentation entirely. I am sure it will make things a lot clearer for you: http://docs.nethserver.org/en/latest/base_system.html#network and http://docs.nethserver.org/en/latest/base_system.html#network-services