If anyone here has PS's "Mournhold Expanded" mod, and you know of the beautiful chapel mesh that comes with it, could anyone cut the large circular stained glass mesh and each of the rectangular stained glass meshes so they're each a stand-alone stained glass mesh?
http://i667.photobucket.com/albums/vv39/RavenWood_2009/asdfg.jpg
Each of the items highlighted in RED should be their own mesh, and no longer part of the chapel mesh.
Can anyone do that?

You may be able to get the result you are looking for by taking the respective textures and pasting them on transparent meshes. IIRC the mesh does not continue into the area occupied by the textures - i.e., the black lines connecting the glass panes are part of the textures, not of the mesh.

Ok... I've never done this before.
So how do I get a mesh into the circular/rectangular shape I want?

You don't need a round mesh as long as it's transparent. I recommend experimenting with http://niftools.sourceforge.net/wiki/NifSkope. It allows you to attach a (new) texture to a mesh. You will likely also want to make sure that the area surrounding the image itself is transparent.

For links to various utilities that might come in handy see http://www.tamriel-rebuilt.org/?p=modding_data/utils.

EDIT - the textures are not part of the mesh. They are stored separately, in the 'textures' folder. You can check which textures a mesh uses by opening it in NifSkope. Irfanview will allow you to open DDS files (if you have the plugins installed, that is).