hi friend
what do you mean first with rejected by quebec !! coz if for exemple being by scq of quebec i dont think that u have anymore right to apply for a federale , coz the first step is being selected for a qualifieted worker ....
If someone is rejected for CSQ, then they don't carry on with that process (i.e., the federal part of the Quebec immigration process) but they can apply for federal immigration from scratch. Just make sure that the reason for CSQ refusal won't be a reason for federal refusal else you may waste your time. But I guess this would be a small probability because you got refused at CSQ stage (rather than the federal stage)....i think.