To remove the Google Play widget, long press it and move it to the top of the home screen over the trash can. You want the widget to turn red, now release it and it's gone.
To ad other widgets, open the app tray by pressing the circle with the six dots in the dock at the bottom. Scroll right until you get to widgets or select he widget tab, select a widget and long press. The N7 will jump to your homescreen, find a space and release to place the widhet on your homescreen.
What messenger widgets are you referring too? Email, envelope with @? Gmail, envelope with red M? Speech bubble with "Talk" in it?
Which ever it is you need to do as above, hold it over the trash can until it's red. If it doesn't go red move it around over the trash can until it does go red.