Should I downgrade Python 3.5 to 3.4?
As of today, yes, it is probably best to downgrade to Python 3.4. With Django 1.8, the current release of Django, Python 3.5 is not officially supported.
The 1.9 release of Django will officially support Python 3.5, but that is not a guarantee that your 3rd party libraries will as well. Ensuring that will likely come down to a matter of testing, and checking the compatibility of each of your 3rd party apps.
EDIT: As noted by knbk, Django 1.8.6 did add official support for Python 3.5. However, this does not invalidate the possibility that your other libraries may not yet support Python 3.5.
I just installed Python 3.5 and created a virtual environment with it. Installed Mezzanine (Django CMS) and tried to run the manage.py file and migrate and syncdb etc.
I've been getting constant errors with 3.5 and I think the reason is that the 3.5 have changed some things that Mezzanine depends on. 
Is it a good idea to downgrade 3.5 to 3.4? Or will I have more problems when upgrading later if I don't adapt to the changes now. Maybe a very fuzzy question, but I come from 2.7 and I think a lot have changed.
I don't know what to do :)