I wouldn't trust my advice very much, since it has no experience backing it (using either 'Nids or GKs). But I do have both codices and can see some advantages you can find.
Holocaust does sound like a very useful ability against 'Nids for clearing swarms. Any blast that size helps more than it hurts, IMO.
I wouldn't think that psycannons would be very useful; 'Nids only have one Invulnerable save (If I read the codex right), and it is backed up by a hearty 2+ save. It is overpriced (in my opinion) in the first place (another Grey Knight would be better IMO).
The incinerator, on the other hand, might be useful for the terminators for taking out a hearty chunk of troops before you charge (or for a deep striking unit with a homer; just make sure you don't decimate every troop within 6" or things will get bad next turn). It's AP value will shred through all of the troops, elites (including warriors), and fast attack choices, even with the armor biomorph upgrades. You'll be wounding on a 2+ or 3+ (auto-hit with template weapon), which means things will burn!
Range combat; only the Warp Blast ability of Zoanthropes/Hive Tyrants can slice through Grey Knight's armor. Storm Bolters will cut through most infantry's armor. For close combat, only the really fat creatures, a massive swarm, or genestealers can really decimate Grey Knights, so pick your battles cautiously.
The tougher creatures (such as a pumped-up Carnifex or Hive Tyrant) seem like they would be easy to crack using a deep-striking (tele homer-wielding) Grand Master Hero (probably with a regiment to increase his survivability). 4+ (Tyrant) or 3+ ('Fex) to hit, 4+ to wound, and ignores armor saves. The Hero goes either first ('Fex) or at the same time (Tyrant). Then use the force weapon ability and kill the beast outright. Chances are, the hero will survive and the big bug will be decimated.
Don't forget the Orbital Strikes. It gets rid of one critical piece of cover your enemy's swarms can use to their advantage. They aren't that expensive, in my opinion.
I'd think an army of tons of Grey Knights troop squads and a deep-striking Grand Master Hero with a Termie regiment. Maybe some vehicles, but probably not (Venom Cannons can easily eat through those, since they can get multiple S10 shots on a 'Fex). That would be probably the best in my opinion, tactically.