I take a multivitamin plus vitamin c, d, all b's with niacin and fish oil, calcium, red yeast rice, should I take the vitamin magnesium with these & why?

I don't like it. Truth be told, I don't like combining Niacin with the red yeast rice as they can both have toxic effects upon the liver. I hope you're having your liver blood enzyme levels monitored on that combination. Adding Magnesium to all of this is not my concern as I don't see any issue in that as long as your doses of all these other medications are not excessive.
It depends. First of all, magnesium is a mineral, not a vitamin. Vitamins are not a substitute for a healthy diet but it is reasonable to take supplements for prevention. Most people don't need calcium supps- evidence suggests this increases risk of atherosclerosis! i don't advise red yeast rice for most. Magnesium deficiency is common- see my comment for a link to see if you have symptoms of low magnesium.