I am not really capable of the more complicated things about the setup of this tank, but will only say that I think you can easy have 6 in the tank – from the fish point of view, I would think maybe even more would be better. But if you wish to keep it very ‘maintenance-free’ then it might of course not be a good idea.
As I said I know people in Denmark has been quite succesfull with these fish, but I cant locate these people, – so I could only refer you to a danish forum site and you might be able to google-translate a bit, or at least look at the images 🙂
http://www.akvarieviden.dk/forum/Regnskovs_biotop_akvarium-2241
http://www.akvarieviden.dk/forum/Regnskovs_akvariet_del_2-2280
This particular tank even had some parosphromenus in it.