Vuk seems to be referring to large signal s-parameters. However, it is also possible to apply true small-signal s-parameters to mixers. SpectreRF supports this with its PSP or periodic s-paramter analysis. In this case the s-parameters are like normal s-parameters, except the signal is a different frequencies at each port.
SpectreRF also provides the PAC and PXF analyses, which are also true small-signal analyses.
So yes, it is possible to have a small signal model for a mixer, but you must somehow account for the frequency translation.
For information on iIP3 and CP1, try
http://www.designers-guide.org/Analysis/intercept-point.pdf.
-Ken