Why not use them? They work fine for GAE. Of course, that may be the reason not to, but at least the map and g3d ones haven't been used (yet).
They should look fine when resized because they do for GAE. No harm in sharing an icon, but if you don't want to, you can just use the other two and/or edit the glest one.
In the taskbar, size differs, since they generally will be bigger icons with a bigger task bar (ie: windows 7 by default has a bigger taskbar (but also a lame method where it defaultly hides the text, fortunately changable)). But for the most part, the icons don't matter too much in the taskbar as they do on the desktop/startmenu/fence/file browser.