[Alicamo]

Ya your calcium is probably low atm, you can add calcium if you wish but it is not as necessary for you at this time because you don't have any sizable inverts. KH is a measurement of a buffer, you may want to raise it in the future to prevent a swing in PH. Mostly i would be concerned about PH, Ammonia, Nitrite, and Nitrate. A nitrate level of 10 isn't bad.

You can add a damsel, there a very hardy fish. Just remember when you go to add the next fish to take some steps to avoid a conflict. Re-arranging the rock is a common one.