Try a little experiment. Get some Mag oxide, mag citrate and mag chloride. Take 400mg of the mag oxide 2x day and see if you have any side effects of non-absorption (aka diarrhea). If not, then increase dose until you get loose stools. Then try the same amount with the mag citrate and mag chloride. Increase to the point of loose stools to determine max dose.
I have in the past used cheap mag oxide as a laxative. It works great, it's cheap and it doesn't cause cramping like purgatives do. For supplementation, I usually use magnesuim asporotate or mixed chelate.