No that will do nothing for mobile devices unless you add the meta viewport tag to the head of the document.
Without the viewport tag the site will be assumed to be 980px (or thereabouts) and just shrunk smaller until it fits. The 767px media query will not apply as it will assume it is a desktop size.
Yes, font size 16px is a standard size. you can use it but about using it you saw font size big or small. you can change the website font size 1 px increase or decrease. 1em = 16px, 2em =32px.
You can use a max-width of 575 for a mobile screen. It is good.
@media only screen and (max-width: 575px) {
.my-class {
font-size: 1em;
}
}