That should be mostly true for the OTU,
Yes, very much so. It is literally how it is stated in the Library Data:
Throughout much of the Imperium today, it is virtually impossible to distinguish Vilani from Solomani and in the middle classes, traditionally both geographically and socially mobile, the distinction has become meaningless as extensive intermarriage has blended the two. The tendency during the Rule of Man for wealthy Vilani to change their names to Solomani surnames has further rendered any wish to make fine genetic distinctions problematical. Only in the Solomani Rim, where many planetary populations have remained overwhelmingly Solomani in character throughout, and in the upper levels of society where individual geneologies [sic] are known with a fair degree of certainty, is there any possibility of making a meaningful distinction. Even between genetically pure Vilani and Solomani, the differences are slight.
Supplement 8 -- Library Data (A-M)
I presume this was before the notion that unmixed Vilani live for umpteen decades more than the average Solomani, because I would not regard that ordinarily as a 'slight' difference.
Vilani & Vargr does add that there are pockets of genetically near-pure Vilani in the area around Vland itself, but even there it only accounts for about 25-30% of the most Vilani-centric worlds in that region. Out in the bulk of Imperial space, about 85% of the population is "mostly non-Vilani ancestry" in stock, while out in the Spinward Marches -- which never had a serious Vilani presence during the
Shangarim Era -- barely 8% of the population can show any significant Vilani ancestry in their genetic makeup.
with a few odd exceptions due to isolated groups which cannot (such as some of the Gene-mods in the former Solomoni Sphere). Certain ones are likely at the "no reproductive crossings" divergence due to 300k±20k years...
Right off the top of my head, we've got the Luriani, those four-armed guys (the Sydites, I had to look it up), and of course the Florians. The gene-modified Jonkeereen and Nexxies in the Marches are probably non-crossable two, on top of the Solomani Sphere experiments.
but canonically Solomani, Sword Worlder, Vilani, Zhodani, and Sylean are all fully interbredable but also each has certain genetic tags. Oh, and there are dental differences across the range. I don't recall exactly which was how many teeth.
That would be the Zhodani, who (presumably) do not have any wisdom teeth, so they only get 28.
The Dynchia either don't have any teeth, or (depending on how you look at it) one each, top to bottom. And that is just one of the weird things about their makeup. I would have put them on the list of non-interbreedable humans (and originally I did), but there is nothing I can find in the literature that supports it, so I left them out. But a Dynchia-Solomani mix would be one weird, weird kid.