Jeeps you had asked if calcium or alk would percipatate because of the large addition of mag. Dons reply was that he figured only the mag would percipatate, which is correct, But in what form. Here is a quick and dirty.
Most of the mag in our tanks are free ions (only bound to water moles), a small percentage is bound to sulfate, bicarbonate, floride, borate and hydroxide. Now only about 10% of mag in SW is bound up by these other elements and it has a very small effect on the total mag, BUT it does have a large effect on those elements.
Mag likes to pair up (ionically bind) with a number of other elements (as listed above). In the case of carbonate (alk) Mag stabilizes it to such a degree that it becomes far more present in SW then it could ever be if mag was not present. So with out this pairing our tanks PH would be much higher and would be more susceptable to large swings. We refer to this as a ph buffer. So in reality its level plays a large role in what it does to the carbonate system (and thus the PH) also it plays a large role with borate which can also effect the buffer system. Then you have to concider the type of salt mix you use, some salts are heavy in sulfates, or borate and so on and so on, so issues could happen their also.
Now pairing and percipatate are two different animals, your question was that you were worried that the over dose would percipatate the calcium and alk. If you can picture adding boith part of lets say bionic to your sump at the same time in the same location, you would be so super saturating it that it would cause both parts to percipatate (ionincally bond) and come out of solution (form a solid) this looks like a cloud in your tank and its very rare as Boomer said as it would take an event such as that to make it occur, it also the reason they sell the products in two seperate containers.
Now if you look at the chemistry and know that 10% of the total mag content in your water gets bound to these other elements and you increase the total content of the mag you are going to also bind up more of these total elements. Even though its not enough to cause percipatate its going to change the over all chemical equation. Now could it be a sharp ph spike that goes up or down for a few hours and then levels itself out?? could be. We had that happen when all these phosphate removers first came out. Folks would add a bunch of it to their system to quickly, they would come back the next day and see no ph spike but their critters were dieing. After someone did an experiment it should that the iron was binding to the carbonate ions and cause a huge PH spike over night but would then level off by the next days.
So the bottom line is that its to tough to factor in all the different things that could happen, thats why we just say take it slow and easy and that way what ever might happen will happen slow enough not to pertain or in time enough to catch it and fix it??
Anyway I dont know if I made it clearer or muddied it up even more lol. But I hope it helps a bit
Mike