Hi,
Unless you have a specific reason to have all internet traffic going through your main server (I can;t think of one right now) then I would recommend that all servers and clients use your router as their default gateway just like your main server does.
Set the default gateway as your router and all machines should get internet access. This will be as secure as your physical network is so as long as no-one can get onto a wireless connection or physically plug a cable into your network then you'll be fine.