Is it possible to change font, size etc on multiple subtitles?

While there's no way to do this directly inside Final Cut Pro, it is possible by using XML. This would involve isolating the subtitles track into a separate sequence, then exporting that sequence to XML, modifying the XML file and then reimporting it.

You would need to do a test and see which parameter you need to change in the DH_Subtitle settings. Take a look at this article as a starting point:
http://www.kenstone.net/fcp_homepage/xml_fcp_hodgetts.html

Alternatively TitleExchangePro from Spherico enables various subtitle operations and supports the DH_Subtitle generator.
http://www.spherico.com/filmtools/TitleExchange/