I'm with Badger; I'd use Carcassonne then go south on the SNCF train/bus line to Axat, from there you can pick up a little tourist train that goes through some stunning countryside to Perpignan then use the mainline train to BCN
Kira

I'd choose Toulouse over Bordeaux aswell. There are daily trains from Toulouse to Barcelona, but they can be expensive unless you have a rail pass. Last time i looked it was €117 return for a 3 hour trip.
Bex, I can't see where, on this thread, you said you were cross-posting.
I made people aware because it's so frustrating to have spent time typing up a detailed reply only to find that someone else has covered the more or less same ground on an identical thread elsewhere. I've had that happen to me.
Venice 1 day?
You won´t have ti,me to sit or get your barings. Let alone enjoy anything.
---------------
DON´T JUST DO SOMETHING - SIT THERE.
Why put yourself on a schedule? You´re on vaCAtion.

This is the newly updated plan. We cut down on the number of cities so we could stay longer and possibly do day trips.
Paris wed 5 dec mon - 10 dec
overnight train
Barcelona tue 11 dec - mon 17 dec
fly
venice mon 17 dec - tue 18 dec
train (4-5 hours)
Rome tue 18 - dec sun 23
train
Florence sun 23 - fri 28 dec
overnight train
Paris sat 29 dec (just to change trains, as flying from Pisa to Amsterdam at this time of the year is crazy expensive)
train
Amsterdam sat 29 dec wed 2 - jan 4
How does all that sound? Any suggestions? Feedback? I'd love to hear it all.
You may get to Venice and curse your decision to spend so little time there. You'll only have 24 hours. At least half of Mon 17 Dec will be spent on getting there from Barcelona and half of the next day (Tues 18) on travelling to Rome. Venice is so beautiful, so unique and such a great place to explore leisurely that spending just 24 hours there is a travesty and, arguably, a waste of travel time.
Remember, a two-hour flight is only the time spent in the air. You have to add a lot more time for getting from accommodation to accommodation, at least another three hours, maybe more. What time does your flight depart from Barcelona?
Many say (and I agree with them) that winter is the best time to visit Venice because of the relative lack of crowds
You´ll be missing lots of fabulous areas, if you go that fast.
Diamonds at your feet department.

Hi, I'm italian and I suggest you to spend one more day in Venice (and one less in Rome).
When in Florence (lovely at xmas) consider to visit Pisa (1h by train) and Lucca (20' from Pisa) in one day, and on another day Siena (1h30' by train).
Consider Pisa-Amsterdam by air: it's about 90 euro (tax included).