Gaius I literally haven't summoned yet, so, I would say no.
Salad I don't have, and if I did get him, I think it's likely he'd stay in my vault, but - the two areas I see him most used right now are DTH and CB. If you're already set on those two fronts I'd probably just leave him unbuilt.
Venus I think has largely lost a lot of her value because of Lydia. They fill very similar roles, but Lydia tends to contribute more to the team pretty much everywhere. The one place I'd consider using Venus would be on CB as a poisoner ontop of a debuffer, but again, I bet you're already set on that front.
Chaagur is interesting, but ... unfortunately, again, I'm not sure if you'll be able to find a huge number of places for him to be truly valuable. As above, DTH and, arguably, arena, but ... if you don't particularly care about either of those then, as with the others, he'll probably remain unbuilt.
But I'm speaking from the perspective of someone who has basically cleared all content in the game, and so the only things which truly interest me are things which will translate into better Hydra output.