The current newton engine supports polygon precise collision. You can check this from the example apps at
http://www.newtondynamics.com/downloads.html, especially the playground. The problem is that the plugin available for GS does not support polygon precise collision. I don't know whether it is the old newton engine upon which the plugin was based or the plugin itself that don't support that feature. Anyway, the new newton engine has a lot of newer features, besides polycollision, that would be great to see implemented in GS, but as long as noone takes the time to build the pluguin/frontend...
As has been concluded in many other posts... we must wait!
As to how you may link/group objects to form more complex collision shapes, using the available plugin and old newton dll, you should have a look at
http://www.newtondynamics.com/downloads/NGDdemo.zip. I found somewhere (maybe [censored]?) the plugin installer which also had the source levels and scripts for this app. If you can get your hands on this, you will be able to see how the 'cylindrical' drums were modelled, for example.