Hi Doitsu,
Your solution is correct but maybe it could be a bit simpler

Let's see:
Code:
@media not amzn-mobi {
span.sc { font-variant: small-caps;}
}
@media amzn-mobi {
span.sc {font-size: 0.7em; text-transform: uppercase;}
}
<p>M<span class="sc">oralist</span> and philosophers have adjudged those who throw temptation in the way of the erring, equally guilty with those who are thereby led into evil.</p>
That works for both, .kf8 and .mobi files.
But if the ebook is exclusively for privately use and the device that we use it supports the .kf8 format, then there is no need to use media queries; just using "font-variant: small-caps;" will be enough