I'm no expert at this but I can tell you from experience that a lot of those bugs do NOT survive forever, or even much more than a year. So you should always repitch when filling or brewing regardless of what you do about cleaning the barrel between uses. I learned that the hard way trying to use the cake from my first batch of sour to start the second.
As for cleaning, I would recomend cleaning as well as you can. What you really want to kill is the acetobacter. There are hundreds of American brewers that think that making a good sour means making vinegar. Screw that. Also, I dont know about you but I wouldnt want stout or barleywine flavor in my sour. My barrel was a brandy barrel that then had Lost Abbey's Angel's Share in it. I rinsed it out multiple times and mixed up the strongest acid solution I could make (using citric acid) and rolled the barrel a bit every few hours for a day. May not be the best way to do it but it worked. 7-8 months later and no signs of anything that I didnt purposely put in that beer, also no wood or barleywine flavor. I should mention that the barrel sat, without a bung in it, for a few months and was leaking when I got it. The rain took care of that for me.