On the 2 or 3 question, I didn't make this up, but I always say "3 where you can, 2 where you need to".
As you'll find out, some of the things you'll want to use will only be supported on 2.x, so you'll probably use 2 more than 3 right now. However, in places where 3.x libraries are available for your project, I'd say jump in and use 3.x there.
As for your bullet points:
The What's New in Python 2.7 document will fill you in on 2.6-2.7 differences. Ordered dictionaries, the argparse
module, and a bunch of 3.x backported features are on the list, plus many bug fixes.
You can install any and all versions you want. I don't follow Debian, but maybe they don't have 2.7 in their repo. You could always install it from source if you needed to, or some Debian user might be able to chime in with another way to get it.
Source code. Lots of it. If you already know the basics, all of these online games and beginner books aren't going to push you to the next level. Find the source of projects you are using, or even the Python standard library, and observe. You might even end up finding bugs and fixing them.
I'm a fan of the ActiveState Komodo IDE. It's not free, but I'm happy with the investment and have been a user since mid-2006.
I always say an IDE is a subjective choice, you go with what feels good to you.
That said, some relatively objective features that make Komodo stand out:
For more info check out the features page: http://www.activestate.com/komodo-ide/features
As for your Slack question, the answer is simple; Slack is by far the most popular chat platform amongst businesses. Our implementation is more or less a proof of concept though, in that there were large under-the-hood changes to easily facilitate sharing through any medium if you're not afraid of writing some basic javascript. We'll be posting a blog on this later today.
I use Komodo IDE which is not free, but they also make a free, lightweight version, Komodo Edit, which doesn't have all the IDE features (like debugging, DB explorer, source control integration and some fancy features like that), which has syntax checking, autocomplete, and shows you method declarations, variables and things like that.
If you don't need it to be in netbeans and aren't able to get those methods to work, I found the Komodo IDE debugging very easy to set up (even places the DLL files/edits php.ini for you), it's not free but does have a free trial that is full featured (and an alpha version that is free until the next release)
My preferred IDE is Komodo Edit on a Windows machine, and I imagine if I became an Ubuntu user I would use the same thing. It's free.
The only downside is it doesn't support debugging. You need to get Komodo IDE for that.
I went from netbeans 7.1 on linux to Kimodo. Index performance better on lower resource machines (netbook) and impressive xdebug integration and looks better but its not free.
If you want free PHP IDE and don't mind installing sun java (esp on linux openjdk not good mix) can't do better than netbeans IMOP.
Read this, it covers the most common tools, libraries, books, etc... for the most common languages ( including C++ and Python ). Invent With Python is a good place to start if you go the Python route, while PyGame is about the only, um, game in town when it comes to Python 2D gaming libraries.
As to IDEs, this is an area I find Python kinda weak. Each time I've used Python I've never really found an IDE that fit well. Komodo worked really well, but was expensive. ActivePython is a good choice and is available in a free edition.
As to C++ IDEs, all my recommendations are in that first link, but it really comes down to your platform. On Windows, I would go Visual Studio in a heartbeat. On Linux, either CodeBlocks or Qt Creator. Qt Creator is actually a pretty amazing C++ IDE.
Pretty simple package I download instantly on every mac/pc dev computer that I hop onto: http://www.activestate.com/komodo-ide Komodo IDE 6 by activestate http://filezilla-project.org/ Firezilla http://www.chiark.greenend.org.uk/~sgtatham/putty/ Putty