1. “Is it possible to estimate the age (or better: date of hatching) of young paros by taking the size as basis? who ag high or low is their growth rate in the first weeks of their life? Are there differences in the various species and forms?”
All definite answers telling a length or a time rate are wrong; they neglect the decisive point. The growth of young Paros depends from the quality of the milieu/the care/the food. I know young Paros which grow extremely slowly, and I know others which grow much faster. In general, the growth of Paros is slower than the growth of many other aquarium fish. But in most cases this is a function of the breeder’s care. We have one very skillfull breeder who manages to get the fastest rate of all, especially in later stages of growth, but in effect his fish sometimes are not the most stabile and sound. Mostly, the quality of the care is at a medium level. If you change water more often, the growth will be accelerated, as it will be if very variable and good food is fed. Important is the space, too. If there is too much space, the young fish will have difficulties in finding food, if there is too little they will suffer from the crowding effect. Mostly, a small space (ten liters in the beginning for about twenty to forty young) in combination with frequent water changing is the best. After about four weeks there should be more space, however.
We do not know of substantial differences between the species and forms. The important factor is that mentioned.
2. “The second, much more comprehensive question is about the determination of the gender in fishes. I heard that there is no chromosomal fixing of the sexes during fertilization, but that the gender evolves in the first few weeks after hatching. Is this true? And – concerning paros – at which age or within what time frame will the gender be fixed? Are all young paros male-coloured and dismiss this colouration when becoming female?”
Indeed: In fish, there is no genetical fixation of the sex. The milieu will contribute to that decision in many respects: temperature, pH, the degree of humic substances are the most important factors. This is a complex feature that cannot be summarized to a simple rule; the process cannot be simplified to naming a number of weeks (see my reply to your first questions). Often, we have broods with 90% of one sex only. To avoid this, try to raise half of the young at lower pH and lower temperature and using less humic substance, and the other half in the opposite way. But this is theory, nearly nobody will follow it.
The opinion that all young Paros are male-coloured first is nonsense. In a way the opposite is true; but this only means that the colours of the males will develop in course of time. I you see an eight weeks’ Paro without any colour, you cannot be sure it’s a female. With some species you will see this development of the colours rather early. If a very young nagy develops blue stripes in the dorsal, it’s a male.