Hello Michael,
Thanks for the advice.
I have changed the order in the Rectangles.xml:

<commandset>
<command>rectcenter</command>
<command>rectangle</command>
<command>rect3pts</command>
</commandset>

But when I started the program, the position of the symbols didn't change. It still highlights the 2point rectangle option. Do you have any other hint?
-moritz