Is adding a second IP address to a router with only one ethernet interface the best solution when more IP addresses are needed on a LAN and the other subnet being used has had all the IP addresses exhausted? Will there be a performance issue if I add 12 more hosts from a different subnet on the same switch where there are 35 other hosts from another subnet within the same VLAN? I cannot use subinterfaces and ISL trunking since the ethernet interface is not a fastethernet interface.
Since you dont have a fast ethernet interface, the only option you have is to use secondary ip addresses.
Adding 1 more subnet to the router ethernet interface as secondary shouldnt be a problem.
One thing to be noticed is that, the bandwidth of the link between the switch and the router will be shared with 12 more additional hosts, during communication between the 2 subnets. ( this case arises when all hosts are communicating simult., which is less likely).
Obviously there is a small percentage of load on the router, because it has to route more packets, not only to the wan, but also between the subnets.