What do you mean by residue? I wouldn't bathe them in the puppy/kitten shampoo, I have never trusted any brand that does that except for one in particular sold at a veterinarian.

Just use the ivermectin paste orally once per day for 2 weeks and the lice will be gone, no bathing needed. It won't hurt the snakes as the ivermectin will more than likely be out of their system a week later.