as vilters mentioned, for conforming clothing you are really better off making transmaps.  its very straightforward, requires no knowledge of grouping/conforming/etc, and any tweaks can be done in seconds in an image editor.

if you try 'cutting' or deleting groups, if you remove so much as one vertice youve just nuked all the JCMs/morphs in the clothing. nondestructive clothing morphs are one thing, actually changing the mesh is another entirely - and most of the time its too much work  (although with poser 2012 you can also transfer weight maps from one figure to another/clothing item, so that makes rough conforming pretty painless if youre working with one of the WM figures).

making a transmap is relatively simple. download the free version of UVMapper. use it to create a template from the UVs. open this template in any image editor - photoshop, paintshop, gimp, whatever. make a new layer, flood fill it with black, and paint any areas of the clothing you want to be visible white, leave any you want invisible black. then simply load this into the transparency channel. you can do more than just hide parts of the clothing - you can make tattered clothing, wet clothing, different materials like mesh, etc with transmaps.

 

for dynamic cloth in the cloth room, trim away. id suggest one of the free modeling apps, since by using it youll take your first steps towards learning how to model. most good dynamic meshes are not quads but are delaunay triangulation, so it wont be a simple matter of area select/delete polys. you may have to use either a boolean subtract, a flatten modifier, or move verts by hand or youll end up with a jagged edge where you cut.
of course you can alternately just use transmaps on dynamic cloth as well.