My philosophy is that you should never run just one Trygon. You should always have at least two of them, and do what you can do to make sure they both come in on the same turn. A single DSing Trygon is a high priority target and will likely die to concentrated fire. On the other hand, it is far more difficult to try and bring down two of them at once, let alone three.
As Travellar said, Hive Commander doesn't stack with itself, and the Lictor/Deathleaper Pheremone trail won't stack until turn 3, assuming he gets on the table on turn 2. With my personal experience, I've found that most of the time, all you really need is a single unit with +1 to reserves, typically either a Tyrant or the Swarmlord, to be effective, unless you are playing a very large game.
It would be worth reading the
Tyranid FAQ. There are some important things in there.
Honestly, I've tried all of the special characters, but I have found that, for the most part, you're better off with other choices. The Doom was broken pre-FAQ, but now he is pretty lackluster. The Deathleaper generally doesn't do enough with his special rules to justify his points cost and the Elites slot he takes up. Old One Eye has a speed problem. He is basically no more survivable than a regular Fex, doesn't have Fleet, can't take a Spore, and can't hide in another unit. The Parasite is mainly a "fun" choice. Against most opponents, you'll be lucky to spawn any Rippers at all, although I feel with the release of the Necron codex, he is going to be more popular as the perfect anti-scarab unit (lol 5 attacks hitting on 3+, wounding on 2+ with S=2xT and Rending, and then a 4+ to spawn Rippers. I'm dieing to test my theory >^_^<). The Swarmlord has the most potential to be useful right now, although you really need to build the list around him.
You're experiences may vary, of course...
