Pat, I wish there was an easy solution. Do you know why Galaxy was surrendered to the shelter?
Be prepared that the Prozac may not help at all, but there are other medications if this happens.
Another though: Since Galaxy is a senior kitty, she probably has arthritis, and she might be painful. Put her on a joint supplement, Cosequin for cats. It is also used for bladder inflammation in cats. It's worth a try.

Best wishes to you, Galaxy and Sparkler!