IB You do not have to convert your currency but if you don't, you are basically short cad. and if the loonie appreciates you lose money. (that is my understanding but call them yourself to clarify)
I am in the U.S. but trade currency so usually I convert to being long Cad but soemtimes I am short. There is a very low fee to trade currency. Mostly it is done in $25,000 lots for the best bid/asked prices and volume. (pro) the bid between bid and asked is like 1/1000 of a cent. You can set up the curency pairs and see the trades live. About $5 million trades every second. There is no option to trade warrants.