CITATION(Noaa @ 21/03/2005, 23:02)
My hotel is by the Arc of triumph.. I guess it's to far skating there..
Arc de Triomphe is about 20-30 minutes away from Montparnasse (where the ride ends), depending how fast you skate. But that assumes you know your way around Paris, and are comfortable skating the streets at night. Might not be so easy in an unfamiliar city.
CITATION
Also, Is it safe (I'm a female) to go by subway alone at midnight..?
I would advise against it, particularly considering you'd be in "foreign territory". When you are not attuned to the local habits and unspoken codes, you might get into situations you'd rather avoid (female or not, by the way).
Maybe your best bet would be to catch a taxi ride back to the hotel ?
Or hook up with some other riders that are going your way and ask them to ride along with you back to the hotel (er, uhm, make sure you're clear on your intentions, lest there be some misplaced expectations...

)
Sorry for the shameless plug, but just in case you do not know about it, there's another mass skating ride in Paris, on sunday afternoons. It's less spectacular (Paris nights are magic) and somewhat slower (more family-oriented), but it's a very friendly atmosphere too.
See:
http://www.rollers-coquillages.org