Thanks Rob_TO.You can add a co-signer later, but only if there is a change in family composition and only from that year forward. So in the tax year you have your baby and family size increases by +1, this will cause a re-assessment of income and for that tax year going forward you can have your spouse's income count as co-signer.
Your spouses income can NOT be added to your income for assessment at family size you submitted app with since you did not have a co-signer at that time.
See here for more info: http://www.cic.gc.ca/english/resources/manuals/bulletins/2011/ob324.asp
Would you be able to answer this:
What would be the application number for the Background / Declaration form? Should this be left empty or is it the confirmation number received with the invitation letter from IRCC?
Also, my address changed recently and it is not the same as the address on the invitation letter from IRCC. Will this cause any problems? Should I include a letter with my application to say my address changed?