I wish I could really help, but my area of expertise also happens to be Robespierre--though I would advise you to take care where he's concerned, since (fantastically untrue, Thermidorian-esque characterizations aside) the difference between a good and bad characterization of Robespierre is more subtle than at first apparent. I don't mean to imply necessarily that you don't, as you say, have "a good handle" on him, just that it's always good to examine one's assumptions, even when one has gotten past the point of indifferently accepting propaganda.

As for Saint-Just, I'll second maelicia's suggestions, and add that it's gotten to be something of a cliché even in the best of novels to refer to Saint-Just as "icy" or some variant thereof. It's best to avoid any descriptors that make Saint-Just sound like the human-ice-cube-machine.

As far as Desmoulins goes, don't make him some sort of Glorious Martyr to Free Speech, because he wasn't that. Nor would I recommend making him incredibly innocent or unbelievably debauched. Don't try combining the two extremes either, because that never works. The best advice I can give where he's concerned is: forget, though it may be difficult, everything you've ever read about him coming from any fictional source. Also, avoid the cliché of making him Robespierre's Best Friend. There isn't really any evidence for that in the historical record, and it's far too overdone in fiction. (But then, I suppose this goes with my previous point.)

...Wow, it seems I had more to say on the subject than I thought. I hope it's helpful.