Can someone possibly tell me how I can add some strings(as in GUILayOut.Labels) between the SelectionGrid buttons, via script? Is it possible or are the buttons fixed and regarded as a whole item?
You can’t. But what you can do is assemble Labels and Buttons freely in your own grid using a horizontal (GUILayout.BeginHorizontal, and EndHorizontal) row of vertical columns (Begin/EndVertical). So long as the Style of the items is the same height, they will line up to form a grid.
You could use GUIContent (text, image, tooltip) and define the look with a GUISkin.