I'll try doing a quick run down of the "fighting" classes.
In order from least to most common (in terms of numbers within an individual swarm), as best I can, with some advice on how to do them (and hopefully pictures!).
1 per SwarmHive Tyranthttp://wh40k.lexicanum.com/mediawiki/images/9/94/Tyranid_Hive_Tyrant_4th_Edition_1.jpgPretty much all of them are different, but probably fireball (or is it possible to fling poison?) and really strong melee weaponry would work. These are basically the leaders of the hive's attacks. You could probably also give these gas attacks or something.
Carnifexhttp://wh40k.lexicanum.com/mediawiki/images/a/a0/Tyranid_Carnifex1.jpgSimilar to Hive Tyrants, but a lot more well armoured and more melee focused. They're also all different. Again, gas attacks might work (spores etc).
Few per SwarmZoanthropehttp://wh40k.lexicanum.com/mediawiki/images/a/a1/Zoanthropemodel4thEd.jpgPsychic attackers. Don't have any idea how to do these.
Tyrant Guardhttp://wh40k.lexicanum.com/mediawiki/images/e/e5/Tyrantguard.jpgUsually defend those "1 per swarm" creatures. Big arse melee tanks, basically.
Plenty per SwarmTyranid Warriorshttp://wh40k.lexicanum.com/wiki/File:Tyranid_Warriors.jpgJust the basic attackers. They sometimes have bioguns, too.
Genestealershttp://wh40k.lexicanum.com/mediawiki/images/0/03/Genestealer_b.jpgFor lots of people, genestealers are one of the defining elements of the tyranid thing. They implant their DNA into other creatures and cause them to have half genestealer children (who look completely human but pass on the genetics) until the fourth generation comes along where the genestealer genetics start to come to the fore once more. I'd suggest just giving them a necrotic poison that spreads on contact. It'd simulate the collapse of the society after awhile.
Ravenerhttp://wh40k.lexicanum.com/mediawiki/images/b/b5/Ravener_model_5th_ed.jpgFast attacking, hit and run snake things. Fast speed and strong melee weapons would do for this.
Metric Shit Tonnes per SwarmHormagaunthttp://wh40k.lexicanum.com/mediawiki/images/5/57/Hormagaunt_minis.jpgMiniature warriors, basically. Imagine goblins but with in built weapons. Lots and lots of these are the best idea.
Termagaunthttp://wh40k.lexicanum.com/mediawiki/images/0/0b/Termagant_minis.jpgLike hormagaunts but with guns.
Gargoyleshttp://wh40k.lexicanum.com/mediawiki/images/7/7f/Gargoyle_model.jpgLike termagaunts BUT WITH WINGS.
Rippershttp://wh40k.lexicanum.com/mediawiki/images/d/d8/Ripper.jpgImagine swarms of locusts. Now imagine those locusts have big arse teeth. Now imagine those locusts do nothing but eat and make more locusts. Give these guys trample:10000 or something to simulate them eating the grass of everything.
Things to RememberPretty much all tyranids (even within a single squad) are different. They have basic "classes", but the idea is that the tyranid swarm adapts it's tactics and is capable of changing to meet the situation. Tyranids are fast breeding but they're usually spawned by spore columns (I think). If you want, make a "queen" class. I don't think anyone will complain. Just make it have massive litters.
That's pretty much everything that's relevant, I think.