How to change shape defaults in keynote?
Yes that is possible - you need to exchange the template file within Keynote.

First, save the presentation you want to use as a the new template with a .kth extension. Keynote will not directly let you do that, so you need to rename the file in Finder.

Within the Keynote.app package, find the folder Contents / Shared Support / Templates. To change the defaults of the "White" Theme in standard format, replace Standard.kth in the White folder.

I have also made a step by step guide with screenshots for the method.