I apologize for not replying sooner. As indicated, it's my first plugin and when I looked at it again this weekend I discovered what caused the 4 characters of unexpected output.
I fixed that in version 0.2 and I hope that this version will be of more use for you since it also adds some options in Appearance > Menus. If you encounter any problems, please don't hesitate to let me know.
Edit: If you could let me know which plugin you tried to install after Category Posts in Custom Menu, or some steps how to reproduce the pluggable.php error, that would be very helpful. Thanks in advance.