If your kidneys are healthy, they will excrete overabundances of elements like Mg. The formulation doesn't matter because the liver and kidneys, not to mention other cells that need the Mg, will strip what they want/need and let the rest pass by in the blood stream. What matters is that you do supplement if you're not getting it from food. So, taking it in smaller doses is okay, but largely unnecessary. You can take the single pill if it's more convenient to do that, and it will be safe to do so once in every 24 hours.

The various formulations have some efficacy specificity, meaning they are better for some conditions than others, although the evidence is not hugely strong. The citrate formulation is the most easily absorbed, and it's dirty cheap. Just watch how much it takes to get your bowels moving so much that it becomes a problem; the citrate formula has that side-effect. I have taken citrate, bis-glycinate, and malate formulations, none with any detectable side-effects.