Can I choose which articles go into a clump?

You cannot specifically select which articles can be clumped, but you can control which pages will appear in a clump.

Clumps "choose" pages that have the most current active date first and if more than one article has the same date, then it "chooses" the pages by 'page rank'. The page ranked 01 will be shown before the page ranked 99.

To control which pages appear in a clump adjust the active date to a current date if you want the page shown or to a past date if you do not want it in the clump.

If you have pages you always want to have in a clump, create a department, make a clump for it, and place only the pages you want in the clump into that department.

This tutorial explains clumps in more detail: