I have that bottle, too, and was surprised to read on forums that people use it on their faces. It stings too much for me to use there but it's fine anywhere else on my body. I put it on my wrists and behind the knees to get it in the lymph system.

Dilute magnesium oil to whatever concentration works most effectively for you. Many people start with a 50%-50% mixture. Adding to a lotion or making your own lotion works well, too. It took me about a month of every day application (of progressively lower dilution) to build a tolerance to full concentration. Everyone is a bit different, and so give it a few months. I now spray Ancient Minerals directly on my skin (except for sensitive areas like my face) without any discomfort.

Regarding using both magnesium oil and Epsom Salts (magnesium sulfate), that's fine. However, magnesium oil (magnesium chloride) is much better assimilated and retained than magnesium sulfate. You might try Ancient Minerals magnesium flakes (available from Integrated Health and other online stores). If you're already using magnesium oil, then taking baths in Epsom Salts will simply provide more magnesium, which won't hurt especially if you are deficient. Ounce-for-ounce, direct application of magnesium oil is the most cost effective because baths typically require more magnesium (of any kind). But, a magnesium bath is an incredibly relaxing experience, and especially good for tight or sore muscles that benefit from the heat.