Answering my own original question here with the REST call I am using if anyone else stumbles on this:
http://localhost/rest/content/fcs/all/news?topn=2&classnames=cms.news&columns=newstitle,newssummary,newsreleasedate,documentId&where=DocumentID IN (SELECT DocumentID FROM CMS_DocumentCategory WHERE CategoryID IN(6))&orderby=newsreleasedate desc
Thanks Juraj for suggesting the WHERE condition.