Will the magnesium from an Epsom salt bath adversely affect the absorption of the selenium if taken at the same time?

No, the magnesium from an Epsom salt bath won’t interfere with the absorption of the minerals. Because magnesium interferes with the selenium in the gut. The interference happens in the gut.

So when you take an Epsom salt bath, the magnesium bypasses the gut and you absorb the magnesium transdermally? That’s one reason why I recommend Epsom salt baths.  

So taking an Epsom salt bath at the same time that you take the minerals is not a problem at all. If you take the minerals orally, and then take an Epsom salt bath, the minerals won’t interact in the gut. And the magnesium won’t reduce the absorption of the minerals in the gut.