[url=http://vignette4.wikia.nocookie.net/destinypedia/images/f/f1/Precursors.png/revision/latest?cb=20141201050439]These are Precursor Vex. [/url] [url=https://www.bungie.net/common/destiny_content/grimoire/hr_images/203115_fe62a375c53a174b3a78d8e8f132f2e3.jpg]These are Descendant Vex.[/url] [url=https://www.bungie.net/common/destiny_content/grimoire/hr_images/203090_3998a61c4e5e2ba238d7c2a66314ed37.jpg]These are Sol Divisive Vex.[/url] All that is known about the Precursors and Descendants is that the Precursors come before the modern Vex we face, and the Descendants come after the modern Vex we face. As far as we know, the Sol Divisive is the only Vex programming to use worship tactics, as instructed by Quria, Blade Transform.

These pictures illustrate my point

We don't know any solid information on The Black Heart. Some believe it to be a worm that Quria captures. Others believe it to be a shard of the Darkness itself. Whatever the case, nothing is known about how the Vex discovered the Heart, nor whether they created or found the Black Garden. As for your quality/quantity point, nothing about the number of these Vex programmings are known, but I would imagine they would be just as large as the other Vex legions. It is likely that the Precursors and Descendants come from different timelines, which is why we only see them in places where timelines converge, namely, the Black Garden and the Vault of Glass. It is unknown if the Precursors or the Descendants have a worship function.