on your first question, you can mix multiple gasses or multiple fluids. you can't mix them together because Flow Simulation doesn't handle 2 phase flow.
on your second question, the developers don't include functionality that they haven't tested against known solutions. that would be a dangerous thing to do. however in the technical reference, there is no specific mixing validation problem. i'd suggest submitting that as an enhancement request. also it is probably worth looking over the equations and methods used in the technical reference to assure yourself that the method matches what you're expecting.
in practice, i've worked with several customers that have used it and it has predicted the mixing pretty close to experiments.
depending on your situation, it is also probably working at the tracer study and particle study depending on what you want to learn.
For your first question, the fluid type means gas or liquid and is also further broken down into types of gases and types of liquids.
For example, in one study you can have several non-newtonian liquids, but you can't select liquids that are both newtonian and non-newtonian in the same study.