Just copying a portion is alright if you're using it for reference. I use authority content on one of my sites, and I use the quotation method in Wordpress to quote certain sections. It hasn't appeared to hurt any of my rankings, although it usually isn't a ton of content. Maybe 100-150 words max.
The site I use quoted content on is a news related blog, so I quote sources, but they are usually brief, like 1 paragraph in length.