The 5-12-26 is a Part A dry nute. It is meant to be mixed with Part B which is a Cal/Nit product. They are kept separate for stability and to increase absorbability. Epsom salts (Part 3 of Jack's 1-2-3) is where the Mag comes in.
If you are running Part A only then you will need a Cal/Mag product to supplement. It works fine.
If you are running 3-2-1 then you should NOT use Cal/Mag. You will not kill your girls but you will drive them to nutrient lock out.
Either way works. IMHO - 3-2-1 FTW!
Hope this helps,
TEX
There is PLENTY of magnesium in Jacks already without adding epsom salt. In fact, if you're using tap (which has additional magnesium) your mag levels may be too high. There is no reason whatsoever to supplement with magnesium unless you're using RO water.