I've done extensive A/B testing with internal links since 2006 and I can say without a doubt that displaying links to related content is a GOOD thing. I can also say that making changes to an existing site, such as by adding 7 new links to every page, may cause Google to re-evaluate your site which, in turn, may affect rankings at least temporarily.
- Make sure the posts really are similar and that they don't just share some common words.
I've tested most plugins that do this extensively, over many sites, for a long time and I can say that the most intuitive(and customizable) at finding related content has been a plugin called "similar posts" by Rob Marsh. The plugin states that it has not been updated in 2 years on wordpress.org but Rob maintains the plugin on his site and it does work well with the latest wordpress - [rmarsh.com
...] I don't know Rob, we've never spoken and this isn't a plug for his plugin arbitrarily, it has really worked well for me.
- Check your page source code for surprises
Many of the plugins I tested stuff a lot of css and author credit text into your page code, take the time to minimize this and combine the css with your main stylesheet for best results.
- Check your database response time for surprises
Because this type of plugin needs to either create new tables to record matching content or do a search on the fly which it may or may not be saving there is a noticeable impact on your database response time. Monitor this closely using a plugin such as [wordpress.org
...] (admin only debugging, turn it off when done) You might be surprised at how many extra calls this type of plugin makes.
- Check the number of links and look for nofollow etc.
Some of the plugins create multiple links to the same page by showing an image with text link for each similar post. Some also have a nofollow option which may be on by default, check the code directly (while logged out of wordpress).
- Consider reducing the number of sitewide links to categories and/or tags (you're not using both on the same pages, hopefully). Similar posts plugins spread any internal link equity between articles which strengthens their profile while reducing that of the categories they are in (a good thing). If the plugin is optimized the category links become redundant and can be combined into a link to an archive or resource page or removed completely.
I would wait for Google to fully propagate what you've done before moving onto any more changes, it may take 90 days or more for a 10,000 page site to fully update in Google, some pages may even take up to a year or more. GWT tells me about old pages that haven't existed in 5+ years sometimes.
Don't panic, make sure things are in order and stick with your overall plan at least long enough to gauge results (2+ months minimum). Good luck.