Some lessons learned form the Foreman running on RHEL6 - the SCL for the language stack, in order to keep the project running on more distributions (RHEL6 vs. RHEL7) is a must: in Rails, the basic problem was, the old Rails version was not running on ne Ruby (comming with RHEL7) while the new Rails version was not running on the old Ruby (comming with RHEL6). That would lead for us to support two different Rails versions, which just wasn't feasible.
Running language stack from SCL brought us a big benefit supporting different os versions. In comparison, the database in SCL vs. the system one has bigger impacts, as was said in the first mail of this thread. Given the conservatism of the SQL ecosystem, I would not be that scared from the Postgres 8.4 not working at all with new Django (although it of course depends how Django developers consider backward compatibilty- however given they support other DB servers as well, I would expect they not using just the latest greates in a specific database and use some common stuff from all databases). Anyway, it's a good idea to investigate this before choosing the right versions.
Although, I don't know much about Django, so my assumption is it's not that different from the Rails in terms of how the community works (of course the Django is much better that Rails though :)