If I'm using a baking soda and epsom salt water mixture will putting the Oscar descale pouch in the water tank overkill? Well it screw with the chemical makeup of the water?

I assume that your recipe is using some form of purified water rather than tap water.
The Oscar pouch will exchange much of the magnesium that you purposely added, replacing it with sodium, defeating the purpose of your Epsom salt. If you want softer water, instead of using the Oscar pouch just use less Epsom salt, which will give you lower hardness and also avoid some of the somewhat undesirable sulfate ion.