Epsom salt

Yes, magnesium sulfate is perfectly fine to use. In general, the only thing you have to worry about mixing is Ca (2+) and Si (4+) ... at normal concentration of waht is applied to plants they are of no concern, but the concentrated stock solutions can form a precipitate if you add them in the wrong order. Just make sure products without Ca and Si are fully dissolved before you add products with Ca or Si and you will avoid any problems with fertilizer. mixing it into soil.. this is probably not considered "slow-release." You probably cannot add enough for an entire grow into 1 pot, because the concentration would be too high with initial irrigations. Stick to top-dressing a certain mount per week and allowing irrigation to water it in or dissolving it into irrigation water. how much depends on how much Mg and S is contributed by other components, so there's no single answer for you...

I was adding epsom, together with worm castings, mycorrhizae and bacterias. But better on bottom 2/3 of pots, for young plants it could be too much for youngsters.