I've continued my relationships and family line experiments that I first reported in the thread linked above. I have some notion that I'll necro my own thread with an update at some point in the future, but haven't organized my notes; for the purpose of this reply I'm not going to pull them out for reference, so feel free to treat my assertions here as unsupported (although I will stand by them).
All my forts now have "socialization chambers" for relationship development. Mostly, I have focused on bloodline preservation and diversification and not eugenics. (I very much look forward to the post-worldgen population continuity aspects of the upcoming release.) I have found no exceptions for the 10-year rule in terms of mate eligibility, either in pairings I've arranged or in historical figures. Without modding dwarves or using a hack like the one provided up-thread, eugenics may best start with the children of migrants and founders, since this provides a sizable population of individuals clustered in similar age groups.
Although the fort used in my first experiment included a number of unmarried migrants of different ages, it was a relatively mature fort (20 years since founding). The subjects who had been born there had developed social skills as dwarf children tend to do; the others had all had at least some time to build up social skills. In a later fort, I found that marriages were a bit harder to arrange among some personalities when the candidates had low social skills. In two notable cases, I paired up dwarves, they spent over a year in their "socialization chamber" and did not become lovers - a rare phenomenon in my efforts. The first couple included a dwarf who "is incredibly frank and candid in dealings with others" and one who "believes that some deception is necessary in relationships with others;" the other paired one who "has a profound understanding of his own feelings" and one who "is mostly unaware of her own emotions and rarely expresses them." The wiki "Personality trait" article lists these as opposite extremes, or close to them, on the same personality trait. I let those dwarves free, and a few years later they had higher social skills, were the only age-eligible partners for each other, were re-burrowed for socialization, and became lovers in 5-6 months.
As for the eugenics question, I don't see any reliable inheritance of personality traits or likes. (I do have one lineage I'm quite fond of, founded by a hunter/weaponsmith and a captain of the guard who both liked crossbows, who had 14 children who all liked crossbows and universally married bowyers, hunters, smiths and marksdwarves, having many grandchildren who often but not always liked crossbows. I think that was a fluke.)
Skin, hair, and eye color all pass on to offspring without dominance. At the time of a baby's birth, it will have one of these traits from one parent. A savescum and reload can end up with the other parent's trait. Hair's trickier. Dwarves who have the "head shaved bald" appearance appear to have children whose hair color is unspecified.
My favored civilization in the world I've played for most of a year has a lot going for it, but is homogeneous in appearance. Their eye colors vary (with aquamarine most common), but they all have pink skin and burnt umber hair (save the bloodlines who've lost hair color). The exception, interestingly, came in the form of the original king consort and his brother, who had pale brown skin, and this has grown to be a sign of royal pedigree in my perceptions of the Handles of Mortality.
To introduce more genetic diversity, I founded a fort with dwarves from another civilization (The Equal Merchants), who are wildly diverse. I had a starting seven with six different skin tones and five different hair colors, plus some interesting eye shades. I then used dfhack to "retire" the fort via adventure mode, and reclaimed it with a party from the Handles of Mortality. A couple of the Equal Merchants married each other, but I paired the rest with Handles citizens. The Equal Merchants retained their original citizenship, incidentally, but this didn't impede cross-civ marriages, and their children are full citizens of the Handles. Not only did I introduce some more diverse appearances (which a eugenics program could take the other direction by eliminating undesired traits), but I also found that religiosity is partially inherited. The children of the Equal Merchant couples alone worship Equal Merchants' gods, plus randomly (?) assigned gods of the Handles of Mortality. Children of mixed-civ couples were born worshiping the gods their parents most fervently followed (from either pantheon) - plus the occasional one from the Handles pantheon they picked for themselves.
I have observed that it seems that *attributes* do seem to derive commonly from parents. It may well be possible to arrange for parents whose kids will favor "unbreakable will" or are "virtually never sick" - but it could be that my observation of trends is coincidental. (Like the Crossbow Clan.) I haven't tried breeding for that myself - my approach is much more "these dwarves are great friends, so I'll pair off a couple of their kids" or "only the progeny of war heroes or nobles/administrators are good enough for the neurotic baroness' children!" or "the children of great artists, or who had childhood fey moods shall marry into the royal family!"
I have no useful info to offer on the breeding of tall/fat or short/emaciated bloodlines.