The Ork version was even better: Any two guns; if they're the same its now twin-linked. For those that wanted melee installing a burna did the trick, you can convert it to that cutting/sword mode.
By Raw, Tome of Blood says "usually consists of". Not always, in other words, but its true that most of the time a bolter is involved. People on both sides of the warp does love themselves a bolter, after all. They're solid, versatile weapons that pack a good punch when legion, so I'd have no issues allowing it for other basic weapon models. Standard Bolter, Standard Plasma, Standard Melta… If you're going to custom-order the bloody thing with a magos that owes you big, Why not Hellgun/Sniper-Rifle? [I can think of a few reasons why not given the alternatives, just sayin]
Now here's a question: What CATEGORY are all those new weapons in tome of blood [and fate]? They're all in one fat ranged and melee pair of tables every time, and nothing is said of proficiencies.
Sure, I've no issue thinking the q'sal stuff is exotic, but there's plenty of far more mundane things that are still considered exotics, so a lot of the options could be anything. What about the Thresher? SP or Exotic? The Lightning Khopesh; power-field or exotic? Is the boiler cannon just a cheap low-tech steam Flamer, or is it an exotic weapon? Combi weapons? Exotic or just two standard proficiencies [the two weapons involved]?
The new splatbooks introduce a lot of awesome rules, but their interactions aren't always explained enough if at all. I'm still trying to get an answer as to integral sniper rifles and Accurate: Its a basic, its fired on single shot, but its Storm; are both getting the full damage bonus?
We indeed could use some clarification as to what can and can't be done with combi-weapons. The rarity doesn't seem to be any worse than "the rarer of the two weapons" here at least, but that's just extrapolating.