I think :first-of-type is only CSS3 (and it would target the first <p>, provided it has a given class, not the "first p with a given class", I believe), so don't expect ePub 2 renderers to support it.
Maybe you can invert the selection. "p.attribution + p.attribution" would target the second and later of the series, which you could use to "cancel" the style you asign to "p.attribution".
But, the best is probably adding an additional class:
Code:
<p class="attribution first"> Harry Summers</p>
<p class="attribution">King of Heartland</p>
<p class="attribution">Companion to the Queen</p>
and then use "p.attribution.first"