Whoever wrote the description of voodoo in Street Magic maybe skimmed the wikipedia article once.
The way Voodoo works, in very short summary, is this*. Voodoo is actually more of a monotheistic religion than most people realize and they worship a single creator god, Bondye. Loa are the servants of Bondye, and are roughly analogous the role saints play in Catholocism (and in fact they were conflated with Catholic saints quite a lot). Bondye is a distant god who isn't involved in the world, and the loa are the intermediaries between Bondye and people. Thus, voodoo priests serve the loa. There are quite a lot of loa with specific histories and such.
According to tradition, priests (there's like 20 different terms depending where you are; houngan [male], mambo [female] are common) are possessed by loa themselves, and zombies are made by trapping spirits of the dead in corpses (specifically, they trap a specific part of the soul that isn't free-willed). However, there's also plenty of legends about the more famous loa having servant or retainer loa.
The obvious ways to fit this into SR magic: Watchers would be the zombi astral (the soul fragment that animates zombies). Either you should have a "family" of loa as a mentor (ex: Ghede, Rada), and summon specific family members as spirits, or you should have a particular famous loa (ex: Marinette, Ezili Dantor, Baron Samedi) as your mentor, and summon retainer loa. The former is less of a departure from how loa is traditionally depicted; houngans are supposed to actually be possessed by "important" loa on a regular basis, not by servants, and houngans generally serve a lineage of loa, not a specific one.
SM seems to be confused about the distinction between a family of loa and the specific loa in that family; eg, they seem to think Ghede is a loa, but Ghede is actually a family (which is further confused by the fact that a bunch of Ghede loa have Ghede in their name, eg, Ghede Nibo).
*Disclaimer: there are a ton of variants of voodoo. This is probably only true for some of them. I'd recommend picking a more specific voodoo tradition (eg, Haitan voodoo), and doing a bit more specific research on that sub-tradition.