the lowest u should be able to descend with the open des should be ur IAA( initial approach altitude) or incase of a visual approach it would be ur circuit altitude, assuming these are set in the FCU Alt window.
Now if u were to descend below it, i would have to be only in V/S or FPA, with the ATHR in Speed mode, since pulling the alt knob would lead to open climb.
(All this assuming FDs are on and are being followed)
airbus has set heights of 500 and 900 for A321 with due consideration to the spool up time.
if you were descending with 1200fpm
which means 20fps, u would loose 160ft at the most while u wait for ur engine to spool up from idle to ga thr.. Worst case scenario...Ofcourse there would be hell to explain to Flight safety but that is besides the point...
hope it helps