[Fyron]

IIRC, the odds for multiple dice with separate outcomes for each are calculated by multiplying the individual odds; i.e., the odds for rolling a Yahtzee (all 5 d6 on same number) in a single roll would be 1/6^5, or 1/7776. However, it's been quite some time since I've studied probability, so I don't remember if that's correct.

That is correct.

Rolling 6 dice at once or 1 die at a time six times makes 0 difference.

There will never be a guarantee of getting that 1 or 6.