I know that on the 330/340 in the case of an APU bleed failing/or an auto shutdown during t/o the packs will go onto to the engine bleeds giving an increase in the EGTs in order to maintain the currnet EPR/N1. I guess the 320 should be the same.
There is no restriction on using APU on a contaminated r/w except if you are going to use the wing anti-ice after lift off. Wing A.I with APU bleed not allowed.