[camlorn]

I can throw a few out here.

Batmud meets almost all of your criteria. You can even play as a duck if you earn it. You have "free levels", and you pick a background (magical, good/evil religious, nomad or civilized (with rather a few odd guilds)) and have 100 levels you can divide between the guilds in your background any way you choose. And, last time I was on anyway, they were between 200-400 online. They don't call it classes, but that's basically what guilds are. Note: duck requires an invitation from another player, if I recall correctly.

Icesus is generally the same system as batmud, at least so far as guilds.

Aardwolf, as previously mentioned, does have multiclassing, but you basically get everything.

Storm hunters, which I've not played in a while, does have multiclassing as you described; do be aware this one is smaller, and may not have as many players as you would want.

Godwars2 also comes close-ish here. You don't get a race--you play as a superneturalis (which I'm probably misspelling), which is basically a god. Multiclassing, not really, but you're not going to get further from human, probably (I could argue multiclassing, for godwars2 but that's sort of trying to put a square peg in a round hole; it'll fit if you take off the corners or bend it...).

None of these are RPI, I'm afraid; I don't much care for RPI.

Can't think of any others at the moment.