In many respects, the difference between intermoduation and mixing is that mixing is desired, and intermodulation isn't.
Generally, intermodulation refers to a 3rd order mixing term in an amplifier stage, where you have two frequencies at W1 and W2 present at the input. At the output, you end up with signals at W1, W2, 2*W1-W2 and 2*W2-W1. This is a problem with closely spaced W1 and W2, because the 2*W1-W2 and 2*W1-W2 frequencies are close in frequency to W1 and W2, and so they are difficult to filter out.
In a Mixer, typically you have W1 present at the input, along with W2 Present at the LO Port. At the output, you have W1-W2 and W1+W2 frequency terms. This is a 2nd order non-linearity, not a 3rd order non-linearity.
Because they are different order non-linearities, you can optimize stages for one behavior or another, because they are governed by different processes. Things that you use for Mixers typically have a strong 2nd order non-linearity, and you don't really care about the 3rd order non-linearity, since you can filter out the intermodulation products, they are different in frequency from the Intermodulation products.
For amplifiers, you generally don't worry about a second order non-linearity, since you can filter out the different frequencies, but the Intermodulation terms can show up in band and cause you trouble, so you want something with a small third order nonlinearity.
Dave