Tacitus didn't do work with Sith, usually, as a rule simply because they had a nasty habit of assuming that they held all of the power in any given relationship. They were a pain in the arse because, sometimes, they had the power to actually back up their arrogance and there wasn't typically a way to test if they did or not before they were suddenly bringing it to bear on you. Most Sith acted the same around non-Force Sensitives and that was with an aloof sense of superiority that was only, at best, partially earned in his opinion.
This though?
This wasn't a deal with the Sith - it was a deal with A Sith. Specifically a new one, one who was wanted by her former Jedi friends and the Rangers for turning her cloak and turning to the other side. Tacitus had heard it said through various mediums that the Dark Side of the Force was a seductive thing that could entice people to do things they usually wouldn't. Which, basically, meant it was a particularly good hooker and the Sith, to his mind, were the punters who got stupid and caught feelings, thought that it wouldn't hurt them.
But it always would.
Still, just one Sith? He could make a deal with just one Sith.
The Sith as a whole? Nah, that wasn't a deal so much as it was an obstruction to be worked around.
Sitting at a table at a backwater cantina on Ord Mantel that he had selected, Tacitus was busy cleaning one of his pistols atop the table. It was a decent cover for waiting for his contact to arrive and it meant he kept a weapon out in the open and in hand. Did he think he would need to use it? No. But the Golden Rule was to be ready for shite to slam sideways in a big, quick, way.
@Cpt Geary