Tolkein does not say if any specific kindred of Elves were captured by Melkor, so I would guess Orcs could have been a mixture of all the kindreds.
Melkor, I think, was capturing Elves before the Valar were aware of them. Once they learned the Elves had come, I believe Melkor stopped trying to capture them or scaled back considerably, being more concerned of the Valar waging war against him to protect the Elves.
Something else I question is why didn't Melkor turn all the Elves he captured later on into Orcs as well rather than making them thralls in Angband? Maybe he:
1) No longer had the power to do so
2) It was pointless as he had enough Orcs already
3) The other Elves captured later were not corruptable? (but this is a stretch)
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