Port Security - You can configure the MAC addresses allowed to connect to a specific port. If you are using a comperatively new switching platform, you can take it a step further with VACLs (Layer 2 ACLs).
To answer the second part of your question - yes, you can connect 2 routers to a switch and put the ethernet interfaces in the same subnet. That would, however, be difficult to manage. You either will have to change the default gateways on half the...
You need to configure a BVI - Bridged virtual interface. Search CCO on BVI and IRB (intrgrated routing and bridging) and see which one best fits your needs.
