Gaius is in my main arena team, but needs to be paired with another bomb champion (in my case that's Warmother) ... pretty effective, if my Arbiter wins the foot race 

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.

That is my problem to be honest, i have cleared all rotations of DT hard now ( seer made DT trivial), i have lydia, i am in G1 tag and i have auto teams for dungeon. CB i run mythfu

Trying to find goals or champs to help me meet those goals is getting hard? But i have 25 rank five food champs already and no-one to use them on.

At this point- just grind for gear essentially? i want siphi and rotos but there arent many others i really want

I have Mashalled. He has gotten me through everything. I see him a one of the best legendaries in the game.

So, I like Salad in Hydra but I do think he falls off quite a bit after Normal/Hard, or can. Auto friendly in the right comps. 

Venus however 100% does some insane work in Hydra, obviously better with Cupidus. Everything about that kit just screams Hydra viable, Buff strip, HP burn and defense down + weaken is just incredibly strong. 

I can't recall if you hate Hydra or not, but just my two cents. Otherwise can happily focus on the gear grind and make food, maybe get ready for a future training tourney that has a champ for winning it, build up for that. 

Out of all those mentioned, I only have Gaius, and yes, he is very useful in G3 and G4 tag arena. Weather you put him in SS or not is your choice, but be aware he will need a lot of ATK, a lot of ACC and an Increase ACC buff for all those high-end teams. Lots of Mythralas and protection set Nekreths out there.

@dthorne - i did consider venus for hydra dont have cupidus though, do you use her?

@tyr - yeh i had considered stoneskin gaius, when i see him in tag, i just make sure to include block debuffs however?  and agreed on mithrala :)