None of those places are too far away from each other, but that’s a lot of train travel. It would be kind of tiring to spend one night per destination.
Toulouse is worth seeing.
You could visit Carcassonne (which also is well worth seeing) as a day trip from Toulouse. Or spend a couple days in Toulouse, then a day in Carcassonne.
Avignon or Aix-en-Provence are also worthwhile.
But if I were you I’d drop a city or two from the middle of your trip.
You could pick stay at one location in the Cote d’Azur (Nice, Antibes, etc) and see all of that area over several days.

I agree with Cil; stay in fewer places and take advantage of day trips to visit most of the others.  Toulon and Cannes are only a little over an hour apart by train.  Nice and Cannes are only minutes apart and the trains run every few minutes.  Drop Cannes from you list of places to stay and visit it on a day trip or as an all day stop en route from Toulon to Nice.  

i’d drop Marseille
if you check out Carcassone, you can just show up in the evening and then you’ll have the place pretty much to yourself
you could stay at the hostel in the Cite or if you decide to spend the day there, check out Sidmums out in the boonies of Carcasonne………a different kind of place. similar to a hostel but better

Enroute from Barcelona to Narbonne, you’ll pass through Collioure, which is very worthwhile seeing, and is small enough that you could hop off the train, spend a few hours, and continue by train (I’ve stayed a few days, and been happy with that time, too).
I’m probably in the minority about Carcassonne, but I think that the best part is seeing it from outside! The first time I was there, I saw the Cit when I arrived by car at dusk, and the view was spectacular. I spent the night and felt that was nothing special. So, my advice would be to definitely go. The new town is nothing special, and you can see La Cit in a very short time—- or you could spend the night at the hostel.
Montpellier is a good destination for you: Lots of students, nice walking. I like Avignon and Arles, and you could see both in a day. If you have to choose, I’d say Arles.
Marseille is a big sprawling port city, with some pretty rough-looking areas, especially between the train station and the vieux port. An option would be Aix-en-Provence.
Toulon is another city that can be a little rough-looking. Very near there, before you’d reach Toulon from Marseille by train, is a tiny town called Sanary-sur-Mer. The station is almost on the beach, and the town itself, while quite small, is very represenative of other towns along the coast. My point is that you could save time by seeing Sanary, and avoid going all the way on to Nice, unless you’re interested in specific things in Nice, such as museums.
If you do save a day or two that way, you could make a stop in Lyon and Beaune/Dijon. Dijon and Beaune are only about 20 minutes apart, and you could see each in a few hours.