«lines without superscripts … to have another line-height than those with these elements.[…] add a css line for p with line-height»

The XHTML + CSS solution for this in not trying to impose a line-heigt for p, but to keep the superscript in the default line-height, like:
<sup style="vertical-align: top;">1</sup>
or some classes to that effect
a<sup class="exponent">2</sup> for a square
2<sup class="ordinal">nd</sup> for second
these class containing CSS "vertical-align: top;" in their definition

In fact I use
.ordinal { font-size: 80%; vertical-align: top;}
so that it works with <span> also

then in TEI
2<hi rend="ordinal">nd</hi> for second

There is no need to edit tei.css
It is better to add the parameter
<xsl:param name="cssSecondaryFile">myowncss.css</xsl:param>
pointing to some CSS adapted to your project