The champer in the house says that imps are, indeed, of the demon family. He also says that whilst you can get slayers for most of the different monsters that spawn, the only really useful ones, in his opinion, are the vermin slayer and the undead slayer. Otherwise you'd be switching out talis every time you switch from one monster to another. Every few seconds or so, in fact. He does, however, suggest using general ones like undead slayers on weapons or books etc. Further advice is that you could pick the mob that gives you the most grief in the higher level spawns, then you could match a tali to that. He says he hasn't tried this, but, for example, you could have a flame slayer tali for the various gargs in a high level imp (semidar) spawn.