Good typography is effective, emotional, and powerful. The right typeface sets the tone and voice for the text. It allows the reader to focus on the content without even noticing the formatting. The typeface influences our behavior and affects how the content is perceived or understood. While outlining typefaces guidelines it is recommended to trust your own judgment in finalizing your decisions. Style the hierarchy of your content and design it in a way that it’s easy for your readers to follow.
There’s really no need to mix typefaces. You can structure your content by simply changing the font size and/or weight and making sure it has several different styles. But if you still insist on mixing them then typefaces can be mixed based on different factors like contrast, familiarity, extremity, or diversity. The simplest way to mix typefaces is by their similar characteristics like shape, height, and proportions. The essential factors that impact readability and legibility are font size and line-height, whereas both are influenced by the x-height of the characters, the length of the line, and the shape of the characters.
Moreover, you want the reader to be able to follow a line and easily make the jump to the next line. Typography should make content easy to read and there should not be confusion regarding where the title is, and where your eye should go next.