How do I change the default font-family for all slides in keynote
You can change the Master Slide to get the fonts you want, and if you’d like this to be the default, then you can save this out as a Theme.

(Caveat: I only have iWork ’11 in front of me, but I believe it's the same, for iWork ’09).

(i) Adjust font for a single presentation


Open the complete set of Master Slides. View > Show/Edit Master Slides.
Find the Master Slide(s) that corresponds to slides that you’ve used in your presentation. Open the Fonts panel (Format > Font > Show Fonts, or the Fonts item in the toolbar).
Select everything in the slide (Edit > Select All, or ⌘A), and then, in the fonts panel, select Courier instead of Gill Sans.
Repeat for every Master Slide you need.
Create a text box and format it with the font/style you want. Then select it and choose Format > Advanced > Define Text for All Masters. 


(ii) Adjust font of a theme for all future presentations based on this theme

Create a blank slide show with the chosen Theme, make these edits on every Master Slide. (Every slide should now have Courier instead of Gill Sans as the default font).

Then save this out as a new Theme (File > Save theme…) to avoid overwriting the default. Call it something descriptive, like Courier Black, and just save it.

Now, next time the Theme chooser pops up, you’ll be able to choose your Courier Black variant, which will have Courier as the default.