1 Can you say breakfast without adding "petit"?
2 What is the difference?
These words are not interchangeable and mean to different things:
petit déjeuner = breakfast (in casual speech, shortened to "p'tit déj")
déjeuner = lunch
I agree. In Québec, they say in an other way:
déjeuner = breakfast (France = Petit-déjeuner)
dîner = lunch (France = Déjeuner)
souper = dinner (France = Dîner)
Yes. When I was a child, some people still used in France the same words as in Québec, but the new terms were already becoming prevalent in the cities (not much in the countryside). Now, I think the new terms are used everywhere in France.
Not in France. It looks like Canadian French differs on this point. Think of petit dejeuner as a little lunch (or before noon) and dejeuner as noon or later.
Déjeuner est lunch petit déjeuner est breakfast
déjeuner : lunch
petit-déjeuner : breakfast
déjeuner : breakfast
diner : lunch