I see, so basically its not the greatest idea to do a Solera in a 10gal barrel? I wonder how small you could get without it being a problem.I'm also not up to speed on brett and what it brings to the table vs other bugs. I mean I have had 100% brett fermented beers before and they were not sour, just funky I would say. And then I have had plenty of sours that say there is brett in there. So I'm curious where the sour comes from, another strain of bugs I assume.
Lacto, Pedio and Brett are a good place to start. You can do it in a small barrel, I had a friend do one in a 15g barrel, but you just need to sample it to make sure it does not get to acidic. A nail through the head makes a simple way to take a sample and not disrupt a pellicle.
I have always wondered if there was a way to lacquer or seal up the out side of a smaller barrel to help minimize the O2 that passes thought it.