Mike,
I'm glad everything worked well! You raised a good point about adding products to existing Web PI tabs. This is not allowed, that's why you have to define a new tab to be able to display your custom products. Thank you, I have updated both articles with this information.
As for a feed not being displayed/being rejected - you can always check the logs under %userprofile%\AppData\Local\Microsoft\Web Platform Installer\logs\webpi for a detailed error message.
As for generating hash of an installer file, probably the easiest (and free) utility to use is File Checksum Integrity Verifier. Also added this to the articles.
Finally, when Web PI is running and you make changes to the feed, you need to restart Web PI in order for it to reload the feed.
Thanks for your great feedback!