on WinrezLT directory there is a program called DTXviewer, you can use it to open DTX files, but the output texture is the one that appears on dedit's texture viewer/editor.
To avoid confusion i would ignore the .rez file for now. Just go to the directory of the project you are working on dedit amd go to "skins/characters/alien" and copy the dtx files from there (drone head legs and torso). Then on your main avp2 game directory create one called "custom/skins/characters/alien" and paste there the dtx files you just copied. On the game's launcher command line just put "-rez custom".
Its a much simpler process and you dont have to modify a .rez file evertime you do something to the skins.