1) Do you mean musical composers? Because I'm not aware that anyone other than Michael Bross did any music for them.
2) In Paramonia and Scrabania they were sent to find wildlife for Rupture Farms, and are still outposts used by the Magog Cartel. They are sought after by sligs because no one will catch them sleeping on the job. The Vaults are part of Necrum, which during AE was under the questionable management of the Magog Cartel. The would have been there to guard the slaves and to scout out the area, when they trapped the spirits in the ghost traps. You don't actually see them in the vaults (if my memory serves correctly) other than in the level ending sprint, where the mines had actually expanded partially into the vault.
3) I don't think that they are forced to do it, but those jobs are what appeals to those species the most. Glukkons like moolah but can't do any manual work, so they seek out management of profitable coorporations. The sligs are not very smart, but are sold by Skilllya to the Cartel, and so are employed as guards and yes-men (who befoul anything more than a menial task), and Wolvarks are similar. The Vykkers successfully found a way to make their sadistic tendancies pay by enterring vivisection (I would have suggested dentistry). There are exceptions: Headley Vykkers is a Auctioneer, Lulu's Valet is a... well, a valet, and previously hosted the sadistic industrial show Name That Trauma! and there is also the Newscaster slig for M.O.M.
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