No worries. Trans fluid has more detegerent in it than anything. So long as it's clean trans fluid, it probably won't stain. Getting that piston down is a bit challenging since you only have access to 1 of the 3 O rings. Hopefully you didn't just hammer it on down in there. You can push while working the 1 small center seal but if you hammer it down, you're likely to have split a seal and Low 1 or Reverse won't hold as good as they should. You can also air pressure test that before you go any further to make sure. Flip the case over and one of the holes right under the back of the pan is the one you'll hit with about 30psi and notice what the piston is doing after you have the snap ring in place.