Good to take calcuim with Vit. D and Vit. C. Both increase absorption.
Never take with Iron. They block each other.
I do, but then the brand I take is organic with a 2:1 ratio of Calcium to Magnesium (which the former needs in order to absorb properly), so there’s no question of any block or interference.
No problem whatever. Multivitamins contain calcium so taking some extra calcium is OK.
If you take mega doses of calcium then you will also need a magnesium supplement to balance the calcium as the two interact and too much of one can cause a deficiency of the other one due to preferential absorption of the one in excess at the expense of the other one.