These are the races found in my Quicksilver novel.

Humans

Pretty self-explanatory.

Elementians

Trolls
Goblins
Wood Elves
Water Elves

Each of these races is human derived. The nymphs of the elements are tangible enough that they can mix with humans---though they are still creatures that are too vague to qualify as races---and have created these new sorts of humans. Water elves come from naiads, wood elves from dryads, goblins from earth gnomes, and trolls from stone golems. Each race has its own traits, but is still part of the human race, no matter how strangely diverse they are.

Therians

Therians are also humans. Each individual can shift into one of various animal forms.

Snake
Wolf
Otter
Toad
Fish
Moth
Raven

At present, I do not know their origins. They can mix with humans and elementians, but will only have one animal form each individual can assume. They are fewer than other races who avoid them due to their reputation of being the servants and comrades of vampires.

Vampires

Vampires can be any of the humanoid races, but lose their specific shifting or elemental skills when they gain those of the vampire. Vampires come to be when they are killed by the bite of another vampire, or when they study magic to the extent that they must cross the vampiric threshold in order to gain more power. This process still involves dying. When gathered in great numbers, the mystifying influence they have multiplies and greatly increases their sphere of diabolical influence.

Dragons and Gryphons

I have had the most trouble with these. I'm not certain if their origin is human or animal, and as such, I'm not sure if they should have elemental races themselves. Both are sentient and very intelligent races, equal in social standing to the others, though much more accepted than the Therians. The gryphons vary from the size of human to the size of a lion, while dragons have been found to be even larger, though seldom exceeding the size of a draft horse.

Do people do dragon/gryphon riding?

Only as often as people in this world have piggyback rides on strangers.

You phrase this as if it occurs more often than I'd think.

Are there any scandalous mixings of nymphs and other creatures?

I'm just trying to put it in perspective.

Well, the nymphs are elemental beings that lack a full consciousness and a rather vague physical presence, so it certainly isn't the same as a relation with humans. Yes, the races born of elementals and humans are not born honorably. The elementians that further pursue elementals cause genetic problems for their offspring. People that are more than half elemental develop serious mental disorders, self-destruction, and insanity. The higher the percentage of elemental, the more likely the individual is to have physical deformities in relation to the traits of their elemental parentage.

Snazzy.

Do elementals need to eat?

The dryads eat through their trees, the others do not need food. They are, for lack of better term, the "spirits" of the elements.

What happens if a water source is polluted? A tree has copper nails driven into it? Someone puts out a fire?

Elementals can become grubby if they are dirty or polluted, but if a river dries up or a fire goes out, the elementals often seem to wink out of existence. They return when their life source is replenished, but most do not really die in the ordinary sense.