Wrt Crookshank vs Ostapchuk, I think the important thing is what role they'd be taking.
Crookshank imo might be a better fit in a top 9 role than Ostapchuk, while Ostapchuk fits better in a bottom 6 role currently. If the spot you're penciling them in for is 3rd line, I might lean towards Crookshank if you want some additional offence, but if you're looking at a 4th line role, or a defensive minded presence on the third line, Ostapchuk takes it for me.
Having said that, I think Gaudette is ahead of Crookshank for a top 9 role. He's more proven at the NHL level, put up more points at the AHL level, and has a history with Green. Crook adds more grit, so that might make him more appealing, but neither of them are known for their defensive game and neither of them makes since imo on a 4th line (though Crook maybe makes more sense of the two).
Also, not sure why Gregor was left out (not by you initially but if you're subbing someone in for Ostapchuk or Boucher in the proposed line, he's the obvious choice), he's certainly ahead of Gaudette, Crookshank and Boucher...