I'd say you should start using Git or Hg or any distributed system immediately. No need to worry about getting complete adoption for those to be exceedingly eff

I'd say you should start using Git or Hg or any distributed system immediately. No need to worry about getting complete adoption for those to be exceedingly effective. Svn (or any "centralized only" SCM) is less capable in an environment such as this as it requires everyone to adopt at once. After you have a week or two worth of changes in your repor then you can convert people one by one, and you don't even have to tell people about the repositories you're using until it's time to try to convert them.