So here was my thoughts when I approached this problem:
- How do I give myself the biggest advantage when rushing
- How do I hurt my opponent the most by rushing
In respect to the first thought, shorter maps are definitely better (you were on the right track there), so I chose steps of war. With respect to the second thought I decided to pick a Terran opponent. Marines (the starting Terran unit) benefit a lot more from larger groups than their Zealot and Zergling equivalents so in early game, when we won't have large groups, it makes sense to go for a Terran opponent.
Next I tried to pick a very fast strategy: 8 Pool. I decided on the 8 Pool because I happened to know that you can sustain Zergling production (with a Queen) off of 8 Drones (I don't remember the Zealot equivalent I think its also 8 though).
I build 6 initial Zerglings to put some initial pressure (and stop him from building up Marines) and then I go for my Queen/2nd Ovi. The goal of these first 6 Zerglings needs to be something simple (because I still need to Macro) and needs to stop the Terran from mounting a defense. So I'm going to specifically kill the Marines.
I'm also a pretty good player so I'll even harass the SCVs; however, this is more complicated. I can't let any of my Zerglings die, because I need to build up critical mass and he has enough SCVs to wipe them out, so what I do is a little Brinkmanship, try to lure a few away and kill them off.
You'll notice my reinforcements are starting to come in, and eventually spawn larva will kick in. As more Zerglings come in, I get progressively more aggressive till, finally, I'm chasing down the few remaining SCVs. After some Magical threshold, the computer GGs.