Cadaques from Toulouse

There is bus service to Cadaques from Figueres. Figueres has two train stations, "Figueres" and "Figueres Vilafant". Fast trains (which is what you probably want from Toulouse) stop only at Figueres Vilafant. As far as I know the slower trains stop only at Figueres. The bus for Cadaques departs from very near the Figueres station. It is a very scenic trip. I believe Moventis is the company operating the buses.

There is city-bus service between the two stations in Figueres, but I think it's scheduled to match train arrival/departure times, and if your train arrives late, the bus may not wait. (That happened to me.) I'm sure Figueres has taxis; it would be smart to research their phone numbers ahead of time in case you arrive at Figueres [this should be Figueres-Vilafant] after the bus departs and there is no taxi hanging around. The distance between the two stations is probably about 1-1/2 miles.

I think there is also bus service from Port-Bou to Cadaques, which would cut the number of miles you travel. I don't know that it would cut the travel time. I think Moventis operates the Port-Bou/Cadaques route

wow thanks for the detailed information and tips!

so most likely, I am going to:
Fast train: Toulouse -> Figueres Vilafant
Taxi: Figueres Vilafant -> Figueres
Bus: Figueres -> Cadaques

or
potentially Bus: Port-Bou to Cadaques ran by Moventis

Should I book the train and bus ticket in advance? Just afraid I may miss the departure time in case the train was late.

I imagine there's considerable money to be saved by purchasing the Toulouse-to-Port-Bou or Toulouse-to-Figueres Vilafant ticket well ahead of time if you are sure about that part of your schedule. Or perhaps you can get a good early-purchase deal and have refund/change options. I've never been in the position to buy French rail tickets very far in advance, so I don't know anything about the refund/change rules. You can price tickets for travel tomorrow and the same tickets for the last Friday that's now on sale (probably in early June). The difference is what you can potentially save by committing early. If for some reason you decide to take one of the slower trains, there will be no fare difference (and no assigned seat), so you can happily wait until shortly before you want to travel and buy the ticket then.

I made the Cadaques trip on July 26, 2016, so my experience may not be yours. I didn't buy the bus ticket in advance; I just bought it when I arrived in Figueres on the slow train from Girona.

The Dali Theatre-Museum in Figueres is very much worth seeing for most people, I think. I really enjoyed it, and I am no fan of surrealism. The jewelry collection covered by the same ticket is gorgeous. That's a ticket that does need to be purchased ahead of time, because the DTM gets some bus tours and at least some time slots might sell out. (I don't know whether full days do.)

sounds like you really enjoy the trip there!
Another question is - do you think it is ok for me to take luggage and store it at entrance / check-in of Dali Theatre-Museum in Figueres, then I go visit the museum?

That I cannot answer. I'd say if it's not mentioned as an option on the Dali Theatre-Museum website, you shouldn't count on it. I don't remember any services at the Figueres-Vilafant station, but there might be lockers or left luggage service at the Figueres station. You can Google luggage storage Figueres and see whether something turns up. There are one or two third-party companies operating online that find local stores, hotels, etc., willing to offer luggage-storage service and connect travelers to them. The traveler signs up and pre-pays online. If one of those companies has a deal set up in Figueres, Googling will find it.

One final possibility, which I used once in a similar situation before the advent of the third-party companies, was to walk into a hotel large enough it seemed likely to have a reception desk that would be staffed all day long, place some money on the counter (these days I'd go with 5 euros), smile nicely, and ask the staff person if I could leave my suitcase there for a few hours. The answer (in Italy) was "yes". The Sercotel President and the Hotel Duran would be nearly directly on the way from Figueres Vilafant to the DTM. The Hotel Rambla might be a possibility on the way from the Figueres station to the DTM. The Plaza Inn is close enough to the DTM it could work no matter which train station(s) you were using.

thanks for the info! I found the museum can store luggage before the entrance.
Now i just need to make sure I could commute from Toulouse to Figueres on that day.