You need a pool ready to go at launch, because the only way to come anywhere near your target time per block at launch is for the difficulty to be so high that small miners cannot realistically solo-mine. That is partly due to how huge the large farms are compared to a small miner and partly due to how many large farms will jump on, and, provided a pool is already present at launch, also the number of small miners who will jump on.
Higher difficulty without a pool discourages small miners from jumping on, so mostly it is the large miners a high difficulty needs to be tuned to, so if no small miners try (they wouldn't get blocks anyway at low difficulty their blocks would be orphans) maybe not quite as high would be needed, but still probably at least 0.7 or 0.75 and maybe if a pool is ready then 1.0 might give at least a "we tried, we really did" kind of look to the launch.
(Though likely even 1.0 would still result in blocks coming much faster than your target time because miners of all scales will all be able to mine nicely yet possibly maybe there wouldn't be as massive a horde jumping on your coin as happened for example with Primecoin (for which just one of many cloud providers saw 18,000 people sign up for accounts to mine the thing, and even despite Sunny trying to make it difficult enough still managed to get to a point where blocks came so fast orphans were starting to proliferate for a while.))
Having a pool in place means it does not matter if you start the difficulty too high, because even if it is too high for the largest farms to solo-mine they too can join the pool if it is too high for them.
So, try to make it way to high, thus maybe managing to make it not too insanely too-low. But still likely it will turn out to be too low anyway.
Oh also, maybe, to counter the "over 51% on one pool is bad" argument, release the source code of the pool at launch too, ready for others to simply plug in their domain name or whatever (or maybe they won't even need to do that) to have it run for them on their server out of the box too.
I saw a service post recently that offers wannabe coin-launchers a whole package that includes a pool...
-MarkM-
We think alike
I've built a stratum mining pool that I would like to have in place at launch tomorrow, but I'm having an issue with auto payments at the moment which is stalling the public release of the pool. Hopefully I'll have it resolved with some reasonable time before launch so that people can register and create workers. I'll have to admit, it's my first mining pool endeavor, and this mining pool is meant to be a stop-gap (for the reasons that you described) until the bigger pool operators get involved.