Toe overlap has no impact on a track stand unless a) you're leading with the wrong foot/truning the wheel the wrong direction or b) your frame is so tight that you're playing footsie with the fork itself.

As for chain slack, it shouldn't matter once you nail it. If you're doing a lot of weight transfer back and forth, you're working too hard. Once you figure out the One True Way, you just kind of set up, tensio up either the leading or trailing foot and adjust your stand by simply altering the amount of pressure on that foot (but rarely by actually transfering all your weight to the other foot).