I have never seen any kind of study that indicates that DE internally does anything for worms, and plenty of statements that it doesn't.
On the waiting, which I missed, a couple of weeks or so after the Wazine should be fine. The worms themselves are a bit rough on their gut, so I'd be sure they are eating at least something to help with healing. A bit of powdered milk added to the feed should help, though keep in mind they don't make the enzyme to digest milk sugar, so you don't want to give a lot. It may give them a mild diarrhea.
If they happen to have tapeworms, you can tell by little white segments which are visible in the poop, kind of like grains of rice. That requires a different drug or a higher dose of Valbazen.
Here is a good discussion on worming:
https://www.backyardchickens.com/t/732910/deworming-experience-from-start-to-finish/0_20