1) We use both readings, but we use いく in the spoken style while ゆく is more used in the written style. いく sounds colloquial and casual to me, and ゆく sounds a bit more bookish and maybe poetic. 2) 行った, the past tense form of 行く, is read as いった, not ゆった. Likewise, 行って, the ren'you-kei form of 行く, is read as いって, not ゆって.