The Doom of Mandos decreed that all wrought by the rebellious Noldor would one day fall, and all would come to naught. He warned them that they would be betrayed by their own kin, and go through many harsh travails. I think Turgon knew in his heart of hearts that Gondolin would one day fall, he just didn't want to admit it to himself. He was overconfident in thinking that a kingdom protected by him and his House, and the Eagles of Manwë, would stand forever. So in that sense, the fault is partly his.
Against those odds, I don't think that 7 or 8 thousand more would have helped much. Perhaps more men fleeing with Tuor and Idril would have meant that Glorfindel wouldn't have died, but I don't think they could have kept Gondolin from falling. We have to take into account Morgoth's cleverness. He had been preparing for a large-scale assault for a long time, which means that he had enough time to breed an enormous army in Angband. If he also had that time to come with all those dragons, monsters, etc. etc., that army couldn't have been beaten by anything less than the full Host of the West, led by Eonwë himself, probably.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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