For the first argument I present Fleshounds...20cm base move, 5 for a 100. That's half the board covered 1st turn. Next, World Eaters + a detachment of Rhinos (or two). If you want to get funky, how about a Dreadclaw for them. Heck, go nuts and put the whole World Eaters century in drop pods. Give a key detachment very long legs. Buy some CSM bikers. There are a ton of ways for a Chaos army (except maybe pure Nurgle) to get board control early. For most armies fighting HtH with a Chaos army for objectives is not a smart proposition. If you are using a Khorne centric force you'd be mad not to gear your army up to do this....the bonuses are huge, particularly for the Banelord and LoB, and it is just plain good strategy.
(Primarch did a neat article on how to play Chaos that has lots of cool tips too.)
For the second post, yes, the much shootier Slaanesh Knights make a great combo with Khorne. They can absorb a ton of firepower that otherwise would be killing HtH specilaists, and can pick off tough units with lots of PD. If you're not that concerned about fluff, Tzeentch make a nice counterpoint to Khorne too with some quality ranged units. Having said that, the Blood Gods' dudes can go it alone too, leaning heavily on a couple of World Eater centuries and some CSM support cards....bikes, Rhinos and L.Raiders probably.
In that specific battle....yeah, taking the Manta out was important, but remember that was turn 2, Chaos already had the lead (the main point I was making here
). Also, the Manta had dropped its troops, and the sucker absorbed essentially a whole turns' first fire so it had done a decent job just being there. If I hadn't dropped it before it fired it would have hurt a lot, but perhaps not catastrophically so. Our Tau guy likes to be agressive with the big birdie....I'd have it hanging about the baseline vaporising things with its' railguns, but each to their own! Oh, and Kroot are very good in HtH, and Kroot in trees are very, very good.