sure you can, pc1600 and pc2100 are identical stuff, the only difference is pc1600 is only rated to run at 100mhz, and the pc2100 is rated to run at 133mhz
but you can always run them both at 100mhz, or you can try to run them at 133mhz (there is a good chance the pc1600 ram will be able to handle 133mhz)
but you cant have one run at 100mhz and the other run at 133mhz both at the same time
