If "Good evening, my friends!" is an acceptable translation, should "good night my friends" also be acceptable?
Can "Good night" be used as a greeting, and not as a phrase said before going to bed?
I don't think it can be used in that way... Is the Ukrainian phrase "Добрий вечір, мої друзі!" only used as a greeting or could you say it before going to bed or leaving somewhere as well?
Yes, «добрий вечір» is only used when greeting someone, not when leaving.
Before going to bed, or when someone goes to bed, I'd say «добра́ніч», «на добра́ніч» or «до́брої но́чі».
When leaving in the night (without actually going to bed immediately), I'd use a general-purpose farewell phrase, like «до поба́чення».