I agree that you might want to check with an accountant to see what the current rules are regarding this, but this would not be a "room mate" situation if she's your girlfriend. When she moves in, you'd basically be in that 12-month waiting period prior to becoming common-law under the Income Tax Act.
The definition of common-law is:
"Common-law partner
This applies to a person who is not your spouse, with whom you are living in a conjugal relationship, and to whom at least one of the following situations applies. He or she:
-has been living with you in a conjugal relationship, and this current relationship has lasted at least 12 continuous months;
Note
In this definition, 12 continuous months includes any period you were separated for less than 90 days because of a breakdown in the relationship.
-the parent of your child by birth or adoption; or
- has custody and control of your child (or had custody and control immediately before the child turned 19 years of age) and your child is wholly dependent on that person for support."
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/prsnl-nf/mrtl-eng.html
When I worked at CRA, the common practice was that we ignored any financial arrangements during that 12-month period because you were living as spouses. She is not a tenant because of the nature of the relationship ("conjugal"). I don't imagine that practice has changed as the definitions of common-law spouse and rental income hasn't changed since I last worked, but nuances in the interpretation of law might have changed; hence you might want to double-check.
As for the financial arrangements, it's up to you. When my husband and I moved in together before we were married, we each maintained our own accounts and had a common account that our discretionary expenses went into - mostly money for groceries and gas, etc. That way, the mortgages from our places (we each owned houses), taxes, utilities, etc continued to come out of our individual accounts, and then we each transferred an agreed-upon amount into the common account. We did that to see how we would function sharing an account, as neither of us had ever done that before.
It worked well for us, and within a year we had merged pretty much all our finances together.