A bluethroat will eventually (quickly) outgrow a 75g, and will be a risk for attacking small, peaceful fishes when cramped in a tank that's too small (including a chalk bass).
I don't think the chalk bass would cause any trouble, but it might get beaten up by the 6-line and the dottyback.
Also, is there a reason you want two dottybacks? There's a reasonable chance you'll end up with one dead dottyback; they can be very aggressive, especially towards conspecifics and congeners.
Dave