Assign the placeholder to a class selector like this:
.form-control::-webkit-input-placeholder { color: white; } /* WebKit, Blink, Edge */
.form-control:-moz-placeholder { color: white; } /* Mozilla Firefox 4 to 18 */
.form-control::-moz-placeholder { color: white; } /* Mozilla Firefox 19+ */
.form-control:-ms-input-placeholder { color: white; } /* Internet Explorer 10-11 */
.form-control::-ms-input-placeholder { color: white; } /* Microsoft Edge */
It will work then since a stronger selector was probably overriding your global. I’m on a tablet so i cant inspect and confirm which stronger selector it was 🙂 But it does work I tried it in your fiddle.
This also answers your second question. By assigning it to a class or id and giving an input only that class you can control what inputs to style.