Could taking a small amount of magnesium citrate a day help with my benign" palpitations and anxiety?

Worse: It likely will make them worse. Mag citrate is a cathartic. If you get diarrhea, you will develop low potassium which will make your arrhythmia worse. Ask your doctor if it's safe for you (depends on your kidney function) to take supplemental magnesium oxide which is helpful for some people as an anti-arrhythmic. Don't do this on your own!