If the RT outranges the Chaos ships, everything is simple. Just keep out of range and keep hammering them.
If the Chaos ships outrange the RT, everything is very complicated. The tactical FTL is still an overwhelming advantage, but you need to make actual plans. Keep moving, never be still long enough for them to get a shot off. Keep out of predictable patterns. Getting your own shots off is similarly going to be incredibly difficult, but at least you're aiming at what is basically a stationary target. I'm sure the voidpilot, cogitator sage, rogue trader and ship machine spirit can cobble something together. Best to do some practice runs blowing up asteroids first though...

Given that the mercenaries already have their payment, I would bet at least one of them turns tail at the first sign of a real warfleet.