The three "seeder" cultures established in Trek all explain different aspects of the humanoid-alien question. The Preservers were meant to explain Earth-duplicate cultures specifically -- although there really aren't that many that they can explain, since the Iotians were the result of 22nd-century contamination, the Platonians chose to emulate Greek culture themselves, the Omegans are some bizarre anomaly that actually duplicated America and Communist China over 10,000 years in the past, etc. Sargon's people were meant to be an ancestor for humanoids in general, perhaps including Vulcans, although Anne Mulhall pointed out that humans evolved on Earth rather than being seeded, making the resemblance coincidental. I've always assumed that Sargon's people were the ancestors of most of the near-human species such as Argelians, Elasians, maybe Bajorans and Risians and other minimal-makeup TNG-era species. The First Humanoids from "The Chase" were meant to explain the predominance of broadly humanoid features in species across the galaxy, even those that are distinctly different from human, such as Cardassians and Klingons and Jem'Hadar, yet still have humanlike eyes, mouths, body shapes, etc. Rather than being a single species that colonized other worlds, they seeded the primordial soup of worlds billions of years ago with programmed DNA sequences encouraged to promote the gradual development of humanoid features, explaining how species that have clearly different evolutionary histories could have converged on the same bodily and facial plans so many times. Keep in mind that this happened 4 billion years ago and that multicellular life didn't arise on Earth until 450 million years ago, with hominids not emerging until 5 million years ago. So the First Humanoids' plan was very, very long-term.
They get confused with each other a lot, but they all operated on different time scales and used different methods. The First Humanoids lived and died 4 billion years ago, before any other intelligence (other than the Q, who are at least 5 billion years old) had arisen in the galaxy, and they seeded DNA programs into worlds with the long-term potential for higher life. Sargon's people lived 600-500,000 years ago and colonized dozens of uninhabited worlds (or may have interbred with humanoids on some worlds). The one confirmed case of Preserver activity was only a few hundred years ago (the Okudachron puts it conjecturally in the 1700s), making the Preservers a modern civilization rather than an ancient one, and all they did was to relocate existing threatened populations to other worlds. (Personally I suspect the Vians from "The Empath" of being the Preservers, since they had the same objectives.)
In response to Norrin Radd's original question, most of the humanlike aliens we saw in TOS were meant to be aliens; they were only human colonists in episodes where they were explicitly established to be human colonists, as in "This Side of Paradise" or "Operation: Annihilate!" for example. Back then, it was very common in SF (mainly in TV/film but even in a lot of prose fiction) for aliens to be assumed to look exactly or almost exactly like humans. (Consider John Carter of Mars, where the Barsoomian races include four-armed giants but also include people like Dejah Thoris, who looks entirely human except for having vivid red skin.)
And we plagiarized the Omegan-American Declaration, too. 
