Interesting. 
I think it demonstrates there can be different experiences on this, possibly depending on species or how you raise them. For years, I did a good bit of Malawi breeding, up to a few years ago. Considering the generally promiscuous behavior of Malawi species the primary intervention on my part was to select top males or females from a group from which to raise fry and allow others to simply grow out for trade or sale. I kept them in various configurations, sometimes including extra females and juvie growouts in the same tank. Of any of the Malawi species I raised, size differences were basically random or I selected the biggest and best looking to breed in the first place, so not really a factor.
Before and since doing Africans, aside from my Kapampa fronts, I've done new worlds, mostly SA. Again, I haven't personally seen breeding affect growth in any consistent way. I'm not discounting anything in your experience, different species and possibly different circumstances. What I tend to do with SAs I'm interested in breeding is raise them as a group and when or if I get what looks like a good pair I may or may not separate the pair from the rest of the group in some fashion, though if I do I don't normally separate them to a tank to themselves.
Did this with my current red head geos. Nearing a year after I got the original group of fry, out of the original 20 a dominant pair had established itself. I left them in a tank with some wild angelfish, juvie guianacara, few tetras, etc. and put the rest in another tank, including some really nice males, but no particular pairs formed and I didn't see any spawning. The derecho storm last year had me with no power for 10 days and out of all my fish the only ones I had trouble with were the geos for whatever reason, leaving me my dominant pair in one tank and several extra females in another tank. All have grown since, but the non breeding females were all smaller than my breeding female and have remained that way... and they were in the larger tank, actually.
Pretty much the same story with other SAs I've raised and bred similarly, discus, severums, angelfish, geos, guianacara, rams, (I've kept other new worlds but not bred them). Pairs were usually two of the larger, more dominant fish in the group in the first place. Sometimes I've kept the whole group of whichever species together, sometimes not, but I didn't see the pairs slow down growing compared to others from the group either way. But these were mostly species you'd typically grow out as a group, maybe that's a factor.
I think it demonstrates there can be different experiences on this, possibly depending on species or how you raise them. For years, I did a good bit of Malawi breeding, up to a few years ago. Considering the generally promiscuous behavior of Malawi species the primary intervention on my part was to select top males or females from a group from which to raise fry and allow others to simply grow out for trade or sale. I kept them in various configurations, sometimes including extra females and juvie growouts in the same tank. Of any of the Malawi species I raised, size differences were basically random or I selected the biggest and best looking to breed in the first place, so not really a factor.
Before and since doing Africans, aside from my Kapampa fronts, I've done new worlds, mostly SA. Again, I haven't personally seen breeding affect growth in any consistent way. I'm not discounting anything in your experience, different species and possibly different circumstances. What I tend to do with SAs I'm interested in breeding is raise them as a group and when or if I get what looks like a good pair I may or may not separate the pair from the rest of the group in some fashion, though if I do I don't normally separate them to a tank to themselves.
Did this with my current red head geos. Nearing a year after I got the original group of fry, out of the original 20 a dominant pair had established itself. I left them in a tank with some wild angelfish, juvie guianacara, few tetras, etc. and put the rest in another tank, including some really nice males, but no particular pairs formed and I didn't see any spawning. The derecho storm last year had me with no power for 10 days and out of all my fish the only ones I had trouble with were the geos for whatever reason, leaving me my dominant pair in one tank and several extra females in another tank. All have grown since, but the non breeding females were all smaller than my breeding female and have remained that way... and they were in the larger tank, actually.
Pretty much the same story with other SAs I've raised and bred similarly, discus, severums, angelfish, geos, guianacara, rams, (I've kept other new worlds but not bred them). Pairs were usually two of the larger, more dominant fish in the group in the first place. Sometimes I've kept the whole group of whichever species together, sometimes not, but I didn't see the pairs slow down growing compared to others from the group either way. But these were mostly species you'd typically grow out as a group, maybe that's a factor.
