#10 and #4 size washers will match adult and baby ponies. For the clamp and bleeding tail rust, I always just scrub the rust off with a tooth brush and soft scrub with bleach cleanser, rinse it out really well, then shampoo the tail a few times, defrizz, etc. and then let dry thoroughly before putting back into the pony. Unless the pony or the tail isn't dry all the way before putting it back together, you won't have the problem of the rust coming back. *Oh, and I forgot to say, you can reuse washers that aren't horrid, but make sure you scrub as much rust off as possible and that you dry them really well before putting back on. You could even coat them with clear nail polish as a precaution, but it's probably not necessary.
For cleaning so softs, soft scrub with bleach and a tooth brush. The tooth brush will help lift the flocking to fluff it up, and the bleach in the soft scrub will remove any dirt and kill any mold spores. After scrubbing the pony, rinse it out really well and then go over it with shampoo and the tooth brush to remove any residue from the bleach. I have never had a problem with it removing flocking and you won't make the mane crusty by soaking it in oxyclean.