While they say too many crossovers can ruin a fic, that doesn't really apply if you're writing a crack-fic anyway. So you can add as many crossovers as you like, the weirder the better. Picture this:
During the summer after Harry's 4th year, the Ministry tries to get Delores Umbridge into Hogwarts as the new DADA professor. Dumbledore counters by hiring (Crossover Character) to fill the position instead. The Ministry then tries getting her in to teach a variety of other Elective courses (including some that haven't actually been taught in centuries or have only been taught in other schools until now), but Dumbledore keeps finding (Crossover Characters) to fill the slots, and then makes the Ministry boost the Hogwarts budget in order to pay for all of these new Ministry mandated courses. You can bring in as many weird crossover characters as you want while sticking Umbridge in some truly horrible job that'll keep her busy and miserable (by having Dumbledore 'regretfully' fail to fill the teaching position for an obscure course with an incomprehensible doublespeak title).
During the summer after Harry's 4th year, the Ministry tries to get Delores Umbridge into Hogwarts as the new DADA professor. Dumbledore counters by hiring (Crossover Character) to fill the position instead. The Ministry then tries getting her in to teach a variety of other Elective courses (including some that haven't actually been taught in centuries or have only been taught in other schools until now), but Dumbledore keeps finding (Crossover Characters) to fill the slots, and then makes the Ministry boost the Hogwarts budget in order to pay for all of these new Ministry mandated courses. You can bring in as many weird crossover characters as you want while sticking Umbridge in some truly horrible job that'll keep her busy and miserable (by having Dumbledore 'regretfully' fail to fill the teaching position for an obscure course with an incomprehensible doublespeak title).
“I really hope I’m behind this convoluted mess; at least that way I’ll be able to get revenge by doing this to myself. I won’t even have to feel bad because it’ll be all my fault.” - Harry Potter, The Master of Death by Ryuugi.