The thing about the lower castes having 25 grades comes from Warriors of Kerensky. Page 59 is one page where this is explained but its not the only reference to it in the book.
Grade determines access to everything, what quality housing, medical care, and all the other stuff of life. As well as the Clan version of currency, how much of that they get, which they can hold for a period of time before the Clan takes it back. WoK talks about that as well.
I don't agree that there would be no intercaste socializing/mingling (i.e. coupling). I have never seen anything in canon that calls that out as a no-no. I've looked.
There are a couple Clans that have quirks that might suggest this, like the Steel Vipers and their rigid internal structures and the Snow Ravens and their distrust of strangers. But aside from some Clan quirks like that, speaking more broadly about Clan society. I suspect there's lots of social mingling (and coupling.)
In some situations that would be extremely difficult to avoid. There are many situations where a team of people may consist of different caste members. For example a cargo-carrying dropship or a jumpship might have merchant captain and first officer, a crew consisting mostly of technicians, perhaps a medic from the scientist caste, and a couple laborer caste folks for the physical labor tasks. I think in that group there's lots of coupling going on, across castes. And I really don't think the Clan cares, unless there are resulting children.
And I don't think that's restricted to spacecraft. A food production facility run by the Clans. I can see a few merchants in charge (or at least handling bringing in the raw materials and shipping out the produced KerenskyOats cereal), a work force of technicians and laborers, and a few scientist caste individuals, some with more medical training providing a medical staff on-site or at the closest medical facility, others more literally scientists/researchers some doing quality control testing, or whatever else. All in the same rooms, in the same buildings or in the same neighborhood, working side by side. Spending a lot of time together both on the clock and not.
BaldDen's perspective puts some big walls up between the various castes. In his version they have a hierarchy, they do not intermingle socially and coupling is a huge no-no.
That or BaldDen's sticking point is just the power dynamics between two individuals and whether one person is effectively taking advantage of the other, abusing their power, that may be what BaldDen is really getting at, and some version of that, I could see, sure. Someone abusing the power dynamic to get coupling to happen when that other person isn't interested but feels obligated because of that person's power. Aff that could certainly be a thing, and would be bad. Since some percentage of the human race are jerks (IMO) sadly it probably does happen.
But a lot of BaldDen's examples and argument hinge on caste power. They feel very much like just because one person is off the merchant caste, and the other is of the technician caste, one has power (and power that can be abused) over the other. Just like that.. that alone is a enough to create a big enough power differential between those two individuals that if they are coupling, it's abuse, 100% of the time. And I don't agree with that. I do think it's possible for some merchant and some technician of similar grade, doing different jobs/tasks centered around making KerenskyOats at the factory, working in the same place, looking at each other and seeing a peer or near-peer individual, both sweating in the same facility. I do think that's very possible and commonplace in a lot of situations.
And in that situation, I believe coupling happens and it isn't a misalliance, it isn't chalcas.
But if you don't agree with that, that's fine. I am extrapolating. I can't put a finger on a canon line of text from a book that specifically proves the point. I'm taking what canon tells us and drawing conclusions. Someone else could draw different conclusions. I've had many conversations with fellow Clan fans over the years where there are just some sticking points where we can't find consensus and I've learned to try to be ok with that.
