if you are federal immigrants, you have to land out of Quebec (then you can come back later to Montreal), if you first land in Montreal, you have to take demostic flight to Ottawa, making things difficult to go through the immigration office. Better directly fly to Ottawa, I think.
I know somebody landed from Enrope, they first got to Montreal, and their immigration documents are held at immigration office at the Montreal Airport, then they got into Montreal, but later they have to fly to Toronto, but the problem is that the domestic flight to Toronto will not get him go through immigration office at the Toronto airport, therefore, they have to later ask sb. to drive them to the US-Canada border to land as immigrants.
I think you'd better call Canadian Immigration Office (they have a number for calling internationally, or ask sb. in Canada to call the 1-800 toll free number). Ask them directly before you buy the other ticket. Also, flying to Ottawa should be OK as Ottawa is in Ontario, not in Quebec. Some time Immigration Laws changes with time, better to confirm with them first.