[nereefpat]

Yes, people that supplement Ca will usually use CaCl, unless using kalk (to raise both alk and Ca).

The CaSO4 would disassociate into Ca and SO4, just as CaCl would become Ca+ and Cl-ions.

Two part additives like CaCl will add Chloride over time, which isn't a big deal since there is a lot of Chloride in natural seawater. There isn't as much Sulfate, so using CaSO4 over time could be a problem.

I would get some CaCl. It's pretty cheap.