I think even adding 12 more paros 500 liter it is far too big a tank to keep parosphromenus species, – they would do really well in much much less – and as you say the chance of any fry to survive is a lot bigger.
Chocolate gouramies and betta rutilans would certainly make it difficult for fry to survive.
In my opinion 500 liter is too big for parosphromenus, – if you have the opportunity to have large flocks of one species, – then a tank of 50 or 60 liter could do well also.
In nature I think parosphromenus males, though they are territorial, but the territory they occupy is really small and concentrated around a tiny place where they have found a suitable place to make a nest. The male will come out looking for females in a small area around this place.
The bigger the place the more difficult it becomes for the male to attact the females to this place he has found.
Feeding in such a big tank could become a problem as well, as the parosphromenus will have to be fast – and chocolate gouramies are very fast on food too 🙂 ..