This topic is pertinent to myself and a group of friends as well. Here's what we are doing, to give you an idea:
Nine of us are brewing 5-10 gallons each of a belgian style imperial stout with an OG of 1.085. The barrel is a former whiskey barrel and will be 55 gallons.
Our approach to souring will be that everyone is brewing it with Wyeast Ardennes yeast. We'll be filling the barrel completely with that and leaving it for a couple weeks. In the meantime, I'm doing a couple low gravity batches of beer with a Roeselare pitch. My 5 gallons of stout will be fermented on this 2nd or 3rd generation Roeselare cake.
After a couple weeks, we'll be pulling 5 gallons back out of the barrel, just to see what happened to it in a short amount of time, and replacing it with my Roeselare batch. This will act as our sour starter.
As mentioned above, you probably don't want to do your main fermentation in a barrel unless it is a lambic. One of our reasons for doing a single 5-gallon batch as a sour starter rather than souring all of them initially is simply the cost. This way everyone isn't investing in bugs. Just one of us.