So, by floating or fixing, I'm not sure what you mean. Do you mean "word processing" or "page layout" mode? These are the two modes you can use in Pages. You switch between them in File, Convert To Page Layout or File Convert To Word Processing.
Assuming that is what you mean, you will use Word Processing for EPUB format. The whole point of EPUB is that ebook readers can allow the user to change font, size and other attributes. So the text re-flows depending on their settings. Images can be used, but they should be aligned with the text so that they always appear in the same place in the text no matter how large or small the font size. So this is, for example, the format of novels.
The opposite would be to use PDF that specifies a specific design just as if the text and images were printed on paper, but as a digital document. The user can then zoom in on the page, but they cannot resize or place text on a page. You can use either word processing or page layout for it. The latter would be for things like textbooks or such.