i've got one ... paid about $30-40 for it
the pond is ideal, but they'll be ok in a tank for a while.
temp should be 72 or lower, but some people have had long term success with them in warmer water.
the more important thing is aeration ... air stones, lots of surface aggitation, etc ...
and you are correct, they are most likely young whites. or some sort of hybrid. but definitely not a sterlet