Is it permissible to do qada fasts after eating haram food or drinking alcohol? I mean, if a person eats or drinks something haram on Monday, is it permissible for him to do qadha fast on Tuesday or does he have to wait for 40 days after eating it?

This is the sin has to do the Taubah and istighgar. If Aamal for next 40 days is not acceptable doesn’t mean to stop doing ibadaat in 40 days do Aamal and Istighfar and seek forgiveness so Allah accept Aamal after that period. It is not permission to stop aamal.