Personally, I think you want missiles on a SuperInq. They're not that cheap so you need that 3rd attack die to make them worth it. They're obviously pretty good at getting a lock action in too.
Alternatively, to keep the cost down, you can go with Sense or Foresight as talents. The former as a flanker that the opponent won't want to see at R1, making it a cheap and useful tool for forcing their hand. The latter to give you that potential extra attacking edge and a reliable 2 die bullseye.
With just missiles and nowt else, getting a lock without Jendon or SNR can be tricky, so perhaps just nothing at all is worth trying, they're still good value for force, evade and 2 die.
So, for the 2 lists above. Instead of Predictive Shot and Viziers crew options, (as cool as Grand Inqy is), I think you'd get more out of a couple missiles on the the 2 Supers, using Viz to Super 7th Sis, while he's skirting or blocking space. Also keeps him cheap enough to be a less appetizing target and less of a loss if he goes boom. 7th Sisters ability is very good at getting damage through at key times, so she can easily forgo missiles.
As for the lone SuperInq accompanying the x1s and Duchess, I'd probably look at 7th Sis there instead. Same price, loses the action economy but gains an Init and, with an evade action, can be almost as hard to damage. And again, her ability helps mitigate the 2 dice output.
2 dice guns tend to want to be extremely persistent or numerous to have an impact. Either that or they're bringing something other than red dice to the squad.
