For your cat girl tail, could you maybe use a whip (like the Indiana Jones -I think I've seen a black one somewhere or maybe the circus chap from the minifig series 2 collection) and stick half of it in the minifig's torso and leave the other half hanging out. She may look a little taller, though, I think it might leave a substantial gap between the torso and legs
You could fill the gap between the legs and torso with black elastic bands.
Alternatively, the line that comes as part of collectible minifig's series 3 fisherman. If you leave the 'stud-end' protruding out of the gap, you could hang her from other studded durfaces. I'm not too familiar with catwoman, so forgive me if that sounds silly.
Or, you could tie some black elastic bands together to make a tail.
These are the solutions I can think of, from the top of my head, but I'm sure there are better ones! I haven't tried any of them out, so Ive got no idea how they'd look, but I hope that helps anyway!