Starcraft – How to hold off a 2 player 7pool rush

First,

initial rush survival

Spinecrawlers and properly microed speedlings and roaches can really help in pushing back this initial rush. Force the AI to funnel up the ramp and place the spinecrawlers behind gas and/or an evo chamber. If you scout it early, set your speedlings outside your base so when they attack you get an easy flank. Use multiple queens with transfusion to keep everything going. Remember you can use them even on your spinecrawlers.

In regards to

What t3 units complement a hydra-heavy build?

I think there is a fair amount of discussion about this but I personally find a lot of use with broodlords. A tier 3 unit, which is very powerful at wiping a ground army. I prefer to use them in team games as well. Excellent base assualt as well as support.

EDIT: In a recent match between Idra and Drewbie(?), Idra tech switched from Ultra to Broodlord and ended up finishing the matched. Had he switched earlier, it is speculated that he would have much sooner. More power to the broodies!

Forgive me for the quality of this answer I'm on the road.

Unlike a lot of people I really feel that using drones for defense as Zerg is a completely legitimate strategy. Zerg is very much a momentum race, so many top players strategy revolves around: Power drones and build units at the last possible second. Unlike Terran or Protoss, Zerg can produce a very large army very quickly so I feel using your Drones to delay is a fairly legitimate strategy.

However, I dislike opinions, even my own, so let's see this strategy in practice:

This first video is from a pair of Plat level players. You'll notice that by pulling drones off the mineral line he's able to get a very easy surround (which whoever this caster is doesn't seem to understand) and really take them apart with few losses (believe he loses 3 Drones). This delays the 10 pool long enough for his Spine Crawler to get up and really defend him well. The key point I want you to take away here is: Drones can do Damage

This next video shows a Zerg defending against a 2 Gate back in beta (when Zealots were 5 seconds faster). The Zerg player here completely misses on his scouting. Notice how the Zealots are forced back every time he pulls all his Dones, but when he only sends in a few (or the Queen alone) he gets eaten apart. Drones are only really strong in numbers and when they can get a surround (much like Zerglings). The thing he does do well is sending his drones back to mine every time the Zealots back off. This is the second key point: If you're not fighting, you need to be mining. Pulling drones is fine, but don't leave them lying around

This third video is a little more high level. I actually saw this match live and Blizzcon and the experience was exhilarating. Cool here is defending an early Bunker rush by Boxer. He immediately pulls 3 Drones (1 from scouting) to take out the Bunker. Boxer shows some impressive micro and manages to get the Bunker up and a Reaper inside... just in time to get destroyed by Zerglings. This is the third main point: Drones are delaying tactic, you are using them to stall until real units can get out.

This last video is your money video because its exactly what you're dealing with: 7 Pool into Spine Crawler Rush. We see here that the defending player really shows knowledge of all the principles we talked about. When he sees the Spine Crawlers he pulls a large amount of Drones off the mineral line, forcing repeated cancels (every cancel costs 25 minerals). Next when Zerglings come he uses the mineral trick and then backs out into the open to get a good surround on them and really take them down quickly. Finally, all of his actions are a delaying tactic so he can get his Zerglings out.

So what have we learned:

1. Drones can do Damage. Try to get a good surround, preferably out in the open where he can't use buildings to block you
2. When you're not fighting, you need to be mining. When your Zerglings finally pop and you start winning you need to back off and secure your economy
3. Drones are a Delaying action. Zerg is a momentum race and you're just using your Drones to delay until you have enough momentum to win.

FAQ

But Tzenes, what if he just backs off and powers Drones and has a larger economy than me?

Omnes non possunt omnis. To pull off this rush he is making a large sacrifice, this means you have a lot of room to breath. Worry less about him backing off and more about repelling the rush. By the same token, as soon as the rush is countered, stop producing Zerglings. Chances are he can't afford a second rush, so building a lot of units here is just letting him catch up.