I'm afraid in XSL-FO there is no option to "test" for the current font-size and switch to an appropriate action. That's because the XSLT step is separate from the XSL-FO step. In XSLT, you can do xsl:choose to take different actions. But the resolved font-size is not known until the XSL-FO step after all property inheritance has been resolved, and XSL-FO does not have a choose function.
In the XSLT stage, you can test for various ancestor elements that you know produce small type and take action there.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net