I use the discs with my dremmel, I also use snips (something similar to
this). Snips are also very useful if you build any model kits.
They cut through thicker plastics like a champ as long as you're careful and patient.
When it comes to cutting/shaping/sanding etc. always remember that you can always remove plastic, but you can't add plastic back once it's been cut/shaved off etc. (unless you use glue) so I definitely recommend being conservative with your cuts.
A good needle file set is also good for customizing/shaping things. I have
this set and they're pretty decent.