It's my understanding that there are slight variations in the usage isogloss in the South, but this much was explained to me by Tennesseean, long-time Dallas resident:

you: 2nd person sing.
y'all: 2nd person plur. inclusive
all y'all: 2nd person plur. general.

The latter isn't normally jocular.
y'all is addressing an individual plus the people pragmatically grouped with that individual.
all y'all would be, for example, where you have come up to a podium and you are addressing a crowd.
y'all would be suitable if you were addressing 'Bob' and saying "Y'all gotta have those specs in by tomorrow." In other words, Bob and his department must have those specs in.

As for "Hi all", I'm a Canadian and it wouldn't bother me to hear it or see it in an email. I can't remember off-hand, but I've probably used it myself in the past. The expression is sufficiently informal, on the face of it, and self-explanatory enough in construction; and I can't imagine anyone getting their panties in a knot overseeing it or hearing it.