Lion server as advanced gateway

Hello, first of all, I want apologize for mistakes in language.

Today I've installed Lion server at work. This is our first time of using Mac as network server.

We need this server for several tasks:

- file sharing

- web server with different hosts (we are web developers)

- gateway (2 internet connections: 1 - fast and expensive (eth0), 2 - slow and cheap (USB 3G))

- collaboration assist.

Here I need to say, that prevoriously we hadn't server with those roles. We had:

- My office iMac as storage

- there was a separate web server on each developer's computer

- Windows Server 2003 and Kerio Contnrol as gateway

- as collaboration assist we use white office desk =)

Now about problems. First and last server roles are not so difficult, and mostly so I want to ask about third role: "Advanced gateway".

We have LAN for a 10 computers (some of them PC, some - macs), also we have two internet connections (as I said before) and the mission =):

- ALL traffic from 7 computers must go thru USB 3G modem

- common traffic from 3 others computers must go thru eth0

- all traffic to google, facebook etc from those 3 comps must go thru USB 3G modem (drivers was installed, modem works fine)

Is it real to get it works?