Re: Should I pre-mix retarder medium in new jars of paint?
If you stay under the 20% limit, I can't see any reason why you can't. Try it in a small sample of paint to see how you like it (you could store it in a small jar). It will reduce pigment levels and opacity/coverage - but how noticeable it will be in actual use, you can only answer by trying & seeing for yourself. I mix retarder into the paint while it's on the palette, but pre-mixing it might be more convenient.
I'm not totally sure, but I suspect that student-level acrylics already have a lot of glycol retarder in their formulation...because of their soupy feel and slower dry time compared to artists acrylics.
Tri-Art's Heavy Body and Liquid acrylics are awesome but have never tried any Rheotech yet - how do they compare to other student acrylics?
Color is the most important element in painting - except for everything else