Hello „Vale!“, your questions are rather difficult to answer, indeed, for they demand really knowledge and not experience only. Most aquarists however, even the specialists for such fish as Parosphromenus, do not distinguish correctly between the two; they take their experience for knowledge. And I do not know of any serious investigation into that problem. Whether growth in groups of young aquarium fish is influenced by hormones or metabolites is – to my knowledge – not seriously clarified by research, and I can only answer from experience, too.
There is certainly proof that young Parosphromenus normally grow at a different speed, but mostly this is not attributed to hormones or metabolites but to individual differences. Just as in the case of other animals and plants we have individual differences in growing speed in aquarium fish, too. Certainly, there maybe additional causes to be found with fish growing together in the same tank (and the array of tanks that you describe could be taken for one tank, too). But I think it is sufficient to take the “normal” individual differences between organisms that grow faster and others that grow less fast as explanation. Those who grow less fast could nevertheless have the advantage of being better equipped for certain environmental situations. (Of course, there is one exception clearly to be distinguished from this, namely growth in crowded situations. Here obviously inhibiting factors are working that are most likely to attributed to metabolism).
We have no indications of an allelopathy of different species with Parosphromenus. But the base of this experience is small indeed, for mostly the efficient breeding of these species affords keeping in different small tanks. Parosphromenus are no fish for the community tank, especially with breeding in mind. But there is no indication whatsoever of adults of different species inhibiting each other if given enough space and the right water conditions. In fact, I know systems quite similar to that which you intend to construct which function rather successful a long time – if, that is the precondition of course – the fundamental ecologic requirements are correctly given. The genus is in this respect more homogeneous than most other genera, Betta included.
Therefore I suggest that you go on with your intentions. I do not think that you will be disappointed. But – to repeat it – that’s no answer to your biochemical questions. The background to my reply is not knowledge in a strict sense, it’s experience only.