I've actually gotten to where I mix my own potting soil, prefertilizing it in the process. The only commercial unfertilized potting mixes that I know of are mostly peat based, and I wouldn't want to use just a peat mix due to its tendency to eventually start messing with pH.

If you're actually mixing the MG/perlite at 3:2, it's not going to affect them too terribly much if you decide to use it. If you notice symptoms of overfertilization at first, just cut back slightly on the nutrient regimen. After an eventual flush or two, you'll have washed away a good bit of the ferts in the soil, and can creep the dosage back up to where you want it.