I love the shadowlings as separate individuals; who are you most likely to find in maints during a sling round? Thralls. Half the guys you try to convert are gonna be another guy's thralls. Hilarity ensues.
However, I think the best solution would be to make thralling permanent. This would, I think, have two main effects:
1. Thralls can be killed with impunity, sidestepping the cumbersome, difficult, time-consuming process of dethralling. The difference between security catching you, dragging you to a surgery table, obtaining tools, obtaining a professional, and guarding the process; and security catching you and harmbatoning you into a paste, is huge.
In other words, disallowing dethralling actually nerfs shadowlings significantly; to them, the difference between a dethralling and a dead thrall is nonexistent. If anything, dethralling is worse, since that's another crewman to fight.
2. It would hopefully make playing a thrall a bit less appealing, since once you die, you're out of the round. That means less self-antaggers... again, hopefully. At the very least, it adds an edge of danger and salt to being a thrall that isn't just clowning around making Security look bad as they repeatedly try to subdue you; being actually damaged by lethals sucks.