Essentially the idea for an ambush force narrows down to three "classes" of units; assault, support, and security.
1) Assault. These are units going forward to destroy, delay, and distract the opponent. They do not have to be assault units for hells sake.
2) Support. These are units sitting back and providing heavy fire into the kill zone.
3) Security. Normally these provide early warning, but in 40k that can really only be infiltrators and scouts, or for lack of that, bubblewrap units. Outside of that role, Security teams are there to fight off counter-attacks and kill or delay enemy reinforcements. You've all seen the Dirty Dozen, right? Great! That's what these 3 elements are exactly.
The list I posted in the first thread was pretty sickly overpointed, but the core idea was there. Here's my attempt at tweaking that idea into a Tyranid ambush list.
@1997 pts,
hq
tervigon, cluster: 160
elites: 300
hg x2
hg x2
hg x2
troops:
stealer brood+broodlord, poison, implant ~x8: 197
stealer brood+broodlord, poison, implant ~x8: 197
stealer brood+broodlord, poison, implant ~x8: 197
stealer brood, poison, ~x8: 151
fa:
gargoyle brood w/poison, ag x25: 200
shrike brood, rending, deathspitters, toxin x6: 300
HS:
Tfex: rupture, cluster, regen: 295
The Tfex and HGs are the support line; the tervigon is security for the support; gargoyles are security for the assaulters; shrikes and 'stealers are assault. Pretty sure I'm reading the rules correctly that spawned gants are counted as Troops.
Could probably drop a broodlord to bring in a 3rd HG for one of the Elite broods - or pick up some options for the tervigon...