How to add a contact link next to the 'submitted by' user info?
You certainly shouldn't need to hack core for this. There's a few ways to do this, the simplest way is to customise node.tpl.php. For example if you were using the default node.tpl.php you would copy it to your theme folder, rename it "node--yourcontenttype.tpl.php" and then open it up and find the code:

&lt;?php if ($display_submitted): ?&gt;
  &lt;div class="submitted"&gt;
    &lt;?php print $submitted; ?&gt;
  &lt;/div&gt;
&lt;?php endif; ?&gt;


You can then add the code to generate your contact link, something like this (untested):

&lt;?php if ($display_submitted): ?&gt;
  &lt;div class="submitted"&gt;
    &lt;?php print $submitted; ?&gt;
    &lt;a class="user-contact" href="/user/&lt;?php print $uid; ?&gt;/contact"&gt;Contact&lt;/a&gt;
  &lt;/div&gt;
&lt;?php endif; ?&gt;