The swarms do best as combined arms formations, un-upgraded they tend to get kicked around. 300-350 points has been the sweet spot here. Stuff like this:
Code:
TYRANID SWARM (S) [350]
Two Tyranid Warriors, 4 Termagants, 2 Hormagants, 3 Carnifex
TYRANID SWARM (S) [400]
Two Tyranid Warriors, 4 Termagants, 2 Hormagants, 4 Malefactor
TYRANID SWARM (S) [275]
Two Tyranid Warriors, 4 Termagants, 2 Hormagants, 3 Zoanthrope (745.M41)
Keeping your activation count high (3-4 per 1000 points) is how I lean as well. With your SR being so low, you have to assume you'll loose the roll each turn. You need to have 4+ assaults cued up on turn 2 and 3 to help mediate that. So I usually take a few deep strikers to help keep the pressure up:
Code:
LICTOR SWARM [200]
4 Lictor
TRYGON SWARM [300]
4 Trygon
To keep Swarms alive while they close to engage range, you'll also need some shooting. These are some of my favorites:
Code:
EXOCRINE SWARM (M) [325]
6 Exocrine
DACTYLIS SWARM (S) [400]
4 Dactylis
BIOVORE SWARM (S) [225]
8 Biovore
After that, spice it with stuff you want to try. Harridans give you excellent manoeuvrability late in the game if you can keep them alive. The bio-titans are great at assaults and support. The Dom is the same as well, plus throws out good shooting as she closes. Genestealeres will protect you from deep strikers and are excellent one shot assault formation.