Yeah, even if you have hard water I've seen recommendations to dose a mixed gh booster that contains both calcium and magnesium. If there is any deficiency a bit more of the one that isn't lacking won't hurt most plants. If all the water has is, for example, calcium, and you dose everything including calcium the only thing that happens is the water has a bit more tds right after dosing than you might shoot for ppm wise for calcium.
How much plants have available to grow on can be compared with a barrel with the tops of the boards uneven, and perhaps one or two shorter than the rest. The barrel will only hold as much water as the shortest stave will hold, and likewise the plants can only grow as much as the limiting nutrient allows. You would have to get pretty extreme to give the plants issues with to much of one nutrient. The fish might have a say about the high tds though long before the plants would.