[whpalmer4]

If you want your newly created documents to use Monaco 9, do this:

Open Preferences
In the General tab, click Edit New Document Template
In the window that opens, click on the utilities/section drawer button
Click on the Whole Document style
Bring up the fonts menu and select Monaco 9
File->Save

You might also consider swapping the cmd-V keyboard shortcut for Edit->Paste onto Edit->Paste With Current Style which will strip off any other font/size information attached to stuff you paste into the document and use what is in the document instead. You can do this with the Keyboard & Mouse pane in System Preferences.