I'm afraid I don't know about access to early Scottish censuses. Maybe someone else with more knowledge will be able to advise you.
I take it you use the IGI and Scotland's People in your research? Is this where you got the information about your ancestors?
Have you seen the entry for Alexander and Janet's marriage?
Have you seen the entries for
all of their children?
I know it's pot luck what you get on the old parish registers, but sometimes you can get a little more information such as father's names, occupations or places of birth so it might be worth viewing them all just in case.
Is Janet still alive for the 1851 census? Have you looked for Alexander on the 1851 census?

If I think of anything else I'll let you know.
I think this post might be better on the Scotland board (the rootschat Scottish researchers are second to none) so I will ask a moderator to move it for you.
