Well, since nobody else wants to directly address the question and I've already done so to a lesser degree with the Au Ra...
If all you're looking for is validation on whether or not interracial couples are a thing: they are, but are seen as taboo. While men and women may hit on members of the opposite sex regardless of race, actual marriage and coupling between the two is rather rare. There are few documented instances of it in-game. Aside from Hilda, there are no known biracial characters (despite the problems Ishgardian nobility has with illegitimate children) and her mother and sire are both unknown. The quest givers for Wanderer's Palace HM are a homosexual Hyur / Miqo'te couple (ostracized not because of their sexuality, but again because interracial relationships are seen as taboo), and a Hyur (whose name escapes me) persistently hits on Momodi during one of the Postmoogle quests. If I recall correctly there's also a Roegadyn with two Lalafell children in the Gold Saucer who says his wife "gave him" them, though whether or not he sired those children is unknown (adoption is a thing).
Aside from a single half-elf (Hyur x Elezen), though, no half-breeds have been observed in-universe.
Of course, PCs can engage in interracial relationships all they like. It doesn't affect the in-game world, won't alter the story, and we can't have children so [Y] x [Z] interracial characters don't need to be modelled; besides, who would ostracize [a] legendary godslayer(s) because of their personal relationships? Not like the world doesn't literally hinge on us winning every time...
Things are about to get pretty damn raunchy, so hide-boxed for discretion.
If what you want to know is whether or not sexual intercourse is possible between every race, well... that's a big maybe.
Do not assume that their greater stature means the males of larger races have bigger penises than the males of the smaller races. While this is an easy assumption to make, it has no real merit; yes, in real life the larger the species the larger the penis tends to be, but this is only a tendency, not a hard and fast rule. I'm not going to look up specific examples but needless to say it varies. Just look at humans.
The same holds true of the vaginas of the females.
Now, back to the question: "Can [X Male] have sex with [Y Female]?" Like above, maybe. It depends on the size of the genitalia of the couple. Even accounting for variance, if it don't fit... it don't fit. The only alternative is to assume that Lalafell females have proportionally sized vaginas and no male has a penis too big for it, which... might make sex for non-Lalafell women a bit less than satisfying, if you know what I'm talking about. The inverse is also an alternative; all females have vaginas capable of accommodating the penis of every male, regardless of size. This would get really weird following the "greater stature = bigger penis size" algorithm (a female Lalafell's vagina could go all the way up to her throat, f'ex, which would raise way more questions than it answers) and also make sex unsatisfying for non-Roegadyn males.
As to why you'd ponder this aside from wanting to write ero fanfiction I have no idea, but there you go. I'm not a biologist though, so take that for what it's worth.
Anything else?