Hi,
are you designing a standalone mixer or is it part of an SoC? If it is not standalone, I don't think you need to match the output. You should terminate the mixer with its real loading conditions to get an accurate measure of its performance.
In order to find the output impedance, you can run a PSS+PAC analysis. The impedance is found like in the picture below.
Note that if you decide to terminate the mixer using the balun + port, then its even easier to check the output impedance as you can just run PSS + PSP analysis and plot Z or Y parameters.
regards,
Aaron
P.S. here is an earlier discussion on the same topic:
http://www.designers-guide.org/Forum/YaBB.pl?num=1240334878/18