Hello,
We recommend you to read in detail the documentation available on this page: http://www.phpdocx.com/documentation/introduction/html-to-word-PHP
It explains all supported tag and styles. If the style or tag is not explicity detailed on this page, then it's not officially supported.
For example about font-size, the units may be pixels, points or ems...
And a paragraph custom style can't be applied to a chunk of text, only to the whole paragraph.
We recommend you to use strictWordStyles as false with your own template that is a more flexible approach; or use more than once embedHtml to add each HTML content. To set some rPr attributes (font-size, color, position), this is styles for a run of text, you need to set the strictWordStyles as false. There's no need, but some special cases, to set strictWordStyles always as true.
Regards.