you can have a quill rat drop a colossus blade without any problems if you modify the quill rat treasureclass accordingly. There is no limit to what item can be dropped by which monster.
if the treasureclasses "weapXX" and "armoXX" exist, where XX is a number between 03 and 99 and dividable by 3). This is completely handled by treasureclassex.txt (and a little bit by itemtypes.txt, where you can set if a certain type is a treasureclass, i.e.