Technically that puts you in a violation of our forum rules
, as we don't help in self medication and preparation of cosmetics.
But let's stick with the chemistry. Hydrates contain - in their crystal structure - water molecules. When the hydrate is dissolved in water, these molecules combine with the solvent. Solubility is a property of a main component, in your case amount of zinc citrate that dissolves will be identical no matter what kind of the hydrate it is, but the more water in the crystal the more solid will be required. How much more? Pretty easy to calculate from molar masses and known formulas.
Assuming the number you listed (2.6 g of zinc citrate per 1L) is a correct one, 0.1 g per 100 mL should dissolve without problems (no matter which hydrate we are talking about, there is a large margin). If it doesn't dissolve it can mean it is contaminated with something insoluble.