There should be a menu option to change the ruler/guides behavior to "canvas lock".
Meaning:
With this option turned on, when scaling/rotating the canvas, the ellipse should move and scale WITH the canvas. So it stays the same size and place relative to the drawing but not the ipad screen.
When I position one of the guide tools to accomodate a certain form and then zoom into the drawing to work at some detail, I want the guide to be still in the right size and position for the desired form.
(The adjustment of the ruler/guide tool itself would still work the same as now: with the 2finger-gestures inside the tool.)