1) わたしの ボスから きょうは しごとが ないと ききました。 (WATASHI NO BOSU KARA KYOU WA SHIGOTO GA NAI TO KIKIMASHITA) = I heard from my boss that there is no work today.
You can change the word order as below.
2) きょうは わたしのボスから しごとが ないと ききました。 (KYOU WA WATASHI NO BOSU KARA SHIGOTO GA NAI TO KIKIMASHITA)
The above two sentences mean the same thing. However, it seems like きょうは (today) is emphasized most in the second sentence since it is used at the beginning.
3) きょう、わたしの ボスから しごとが ないと ききました。 (KYOU, WATASHI NO BOSU KARA SHIGOTO GA NAI TO KIKIMASHITA) = Today I heard from my boss that there is no work. (Your English translation is perfect.)
きょう (today) is one of time related words that doesn’t require a particle. However, you can add a は to it as in the second sentence when stressing.
We know that the speaker is talking about “today’s work” in #1 and #2. But we are not sure “which day of work” he/she is talking about in #3. It all depends on the context. For example:
Tanaka: あしたの しごとは ありますか。(ASHITA NO SHIGOTO WA ARIMASUKA)
= Do we have work to do for tomorrow?
Co-worker: きょう、わたしのボスから ないと ききました。(KYOU, WATASHI NO BOSU KARA NAI TO KIKIMASHITA)
= Today I heard from my boss that there is no work (for tomorrow).
As for your last question, なかった can be used instead of ない when “today” is already over. For example, you were off from work today and called your boss and asked if he was busy at his store today. He told you there was no customer ☹ Then you called your co-worker and told him;
ボスから きょうは しごとが なかったと ききました。 (BOSU KARA KYOU WA SHIGOTO GA NAKATTA TO KIKIMASHITA)
= I heard from my boss that there was no work today.
Today is over, so the verb is in the past tense. Keep up the good work!