well it depends where you want to be. For example you do an MD/PhD at UofT, and then you want to specialize in Cancer field. You start your residency in Radiology or Oncology or like specialty.. then you participate in research during your residency. Most likely, you will not have to do a Post-Doctoral Fellowship. Since you have an MD/PhD and you are a specialist.
If you do only an MD, and proceed to complete training in Surgery for example, you may take 2-3 years between your General and specialized surgery, and do either clinical or basic research. Canada has a physician-scientist funding program for people who wish to train in research. Or some programs like UofT where you can do a PhD during your residency (prolonged a bit) and than you continue with residency at full time after your phd is complete.
Please don't ask me to provide links for you.. just do google searches.
"physician scientist CIHR etc... " and Physician Stream PhD... UofT... IMS..."
BTW, I would first concentrate on getting accepted.. before being concerned about all of this. You have 7-8 years MD/PhD + 3-7 years residency and specialization before you will need to do a fellowship. That is 11-15 years down the road.