You should be able to disable the DHCP feature on the router to prevent conflicts with the Linux server.
I think that model has only 4 ethernet ports, so you may want to buy an extra switch for expansion. You'll need to plug in one port of the switch into the westell, and the rest of the servers into the switch and the remaining westell ports. That way, they'll all be able to communicate with one another and the Internet.
The default gateway for all the servers would be the IP address assigned to the westell on the office network. The westell will do DNS for internet IP addresses, but if you can set all the servers to use teh Linux server for DNS, so this shouldn't be much of a worry.