I think there's not much of a rule of thumb when it comes to link diversity - if you suddenly imbue your site with good links, such as links from news sites, PR7-8 sites, etc, then those links will magnify your site's authority by many fold, and it wouldn't be degrading in any circumstances.
In the past, I've used 100% links from articles before, and the lack of link diversity did not effect the overall link structure for the sites being developed, provided that the article link source is at an acceptable level.
If mixing links from other sources, you have to be sure to vet your sources, and do not mix bad links to the mix, as those would either do nothing much to the SERP, or if too many of these links are present, it could transfer a penalty to your site instead.