Multivitamin has 100% daily vitamin B12 (6 mcg) and B6 (2 mg), and 50 mg magnesium. Can I still take b-complex drops and slowmag (magnesium chloride)?

Yes. The amounts that are in your multivitamin, so it is safe to take an additional b-complex and magnesium chloride. The question really should be if you need to take these. There are certain conditions for which it is a good idea to take a multivitamin or even higher dose vitamins/minerals/other supplement, but many people don't need to take anything additional over a healthy diet.
Sure, but... Why? Multivitamins and B vitamins and slowmag are generally worthless, which is why they are not regulated by the FDA. However, they can be dangerous to your health. Stop all of them, eat a balanced diet, and invest the savings in a gym membership. Exercise is the only proven way to stay healthy.