Any way to cycle shortcode while specifying category?

Yes, in Settings > Cycle Shortcode there is a category selector.

I plan to add a category parameter to the shortcode soon.

Did that work for you?

This can also be done in version 1.11 like so:

[strong category="1" slideshow show_for="5" effect_for="1"]

I have just applied the shortcode you mention and notice that the styling is completely different than the single testimonial, and it extends off the page. https://www.kschn.lizschneider.net/speaking/

This is with Page and Widget stylesheets loaded.

The formatting is off because the block is wrapped by <code> and <pre> tags. My guess is the cut-n-paste from this page included those tags. Edit your page in Text mode and remove those.

Awesome, that did the trick!