I believe the control panel "file exchange" let's you choose what programs to open with certain (pc) files (extensions).
So, if you make it a habit of puting the .txt after textedit files, than set the control panel to read all .txt files with TextEdit, i think it should launch automatic.
I also think you can "choose & select" files, than say what program to open for that one (like, shoose a simpeltext, than say to open with TextEdit).
It's been a while that i checked thouse things on Os9 but i can check at work tomorrow.
Hope some of this is true though, hehehe