Swarmlord is fun for smaller games, but becomes worse once you cross into games that are more than 1000 points because
A. He can only slingshot one unit a Turn, whereas in 2k you have many more units
B.He's a targetable character with a 300 point "HIT ME" price tag floating over his head. Once you reach 2k, no capable opponent will ever let him live longer than a single turn.
Carnifexes are both good as melee and shooty. Probably better when shooty (24 S6 shots on a 3+ for just over 100 points is rediculous). I run melee myself because its cooler. Once you're running 3+ of the fexes, consider getting Old One Eye for them as well, since he makes them hit on 2s when they charge. However, he limits your flexibility, since they all need to be within 6" of him.
I'd definently recommend getting some kind of ranged artillery unit, you'll always need it no matter how much you want to go for a melee swarm. Your options are:
>Exocrine (lots of shots, highest AP, good damage, low S)
>Tyrannofex (few shots, highest AP, highest damage, highest S, low BS)
>Biovores (lowest shots, does mortal wounds or creates spore mines
>Hive Guard (average shots, average damage, S and AP, but ignores cover and LOS)
I've been running an Exocrine, and/or a Tyrannofex and some Biovores, but I'd personally recommend the Hive Guard. Main problem with Nid artillery is that you're pretty vulnerable to Lascannon Alphastrikes. Some games my Exocrine/T-fex never even gets to shoot since it dies T1. Hive Guard let you hide them out of LOS, optimally surrounded by a wall of gaunts so melee assailants and deepstrikers canr reach you.
Seriously, 6 Hive Guards will win games all by themselves if they live the entire game long. They're pretty much a "choose one unit to delete this turn" button, and the only effective way of killing them is a flyer.
I myself will swap my Exocrine out for the Hive Guard, once I buy another 3 so I can play a full unit of 6.