Apart from the Fallen One, is any indication given in the games that the Xel'Naga have interacted with the Terrans? I would think that they would have been a prime target for the Xel'Naga to investigate while they were searching for sentient life. There's also the question of whether the Terrans could have arisen without some sort of external influence since this seems to be how things work in the game lore.
Though Starcraft 2 has retconned some of the original info we had re: the Xel'naga, the whole reason that Terrans are in the Koprulu sector at all is because there was a computer malfunction in Earth's original expedition.
(Fun fact: The 4 original ships were penal ships, and the Terrans, penal colonists)
Firstly, the Xel'Naga were not looking for sentient life, they were creating it, seeking first a Purity of Form (the Protoss) and then a Purity of Essence (the Zerg).
The Starcraft 1 manual cites the Protoss watching over the Koprulu Sector for a long period of time before the Terrans appeared, and long after the Xel'Naga had left to create the Zerg.
The abbreviated timeline looks something like this:
You'll notice that Terrans arrive in the Koprulu sector after the Xel'Naga get killed off. Since the encounter with the Zerg and Terran are identified as "Humanity's first contact with an alien species", it is safe to say that Terrans have no record of the Xel'Naga, and it is improbable the Xel'Naga had anything to do with the Terran.
Terrans, unlike the other species in or around the Koprulu Sector, which is around where most of the Xel'Naga's work seems to be. It is extremely unlikely that the Terrans have had any known encounter with the Xel'Naga.
From the original Starcraft manual:
So they had not encountered the Xel'Naga prior to meeting the Protoss and Zerg, and as far as we know there are none left to make contact.
For extra credit, here is the manual: http://ftp.blizzard.com/pub/misc/StarCraft.PDF