Lots of good advice about getting it in writing.
(I'm not going to say "don't do it!" bcs I think things like that -can- be really good things in people's lives, even if they can also be a horrible situation.)
My only point would be this:
Whatever you come up with as "rent" (and I think you should charge it), always insist on being "paid." Don't say to yourself, "Oh, she didn't do the yard work, but I don't want to be a nag, and it's not that important, so I won't say anything."
Say, "Don't forget you need to do the yardwork in lieu of rent." Say it right away, and pleasantly, and not like a -mom- but like a landlord.
When people get things for free, it's very easy to start de-valuing them and taking them for granted--don't do that to her. Hold her to her agreement.
But also recognize your limits. Don't make the rent be "you need to save for you." Make the rent be something that truly compensates you for the extra work/trouble/expense of having someone in your home. Maybe it's that your life gets harder because you share your space, but it gets easier because you will now never need to clean the bathroom or the living room (but you still get the kitchen, perhaps--what seems reasonable).
Make the "rent" also be something that's not a token, something that's difficult enough that she feels she has truly earned the right to live there.
Also, when you sit down with her to work out rent (or "rent"), and house rules, and some dynamics ("let's each have time when we can ignore one another"), discuss this openly. Say, "it's really easy for people to take stuff for granted, so how can you and I be sure that you always value your right to live here, and things don't go sour?"
Drag all those potentially ugly and unpleasant things out in the open *now*, while they are not actually affecting you two, and discuss how you'd handle them if they -did- happen.
Also thing of all the things you might do that could sour things. What if you get to "mommy" acting? How would you like her to handle that? Develop "code phrases" or simply create a pathway that you can use to bring up a topic like that (much the way thinking through what you'd do in a disaster actually carves neural pathways in the brain that mean you react more rapidly and with less panic when disaster does strike).
And another thing to consider: Insurance. Not health insurance--homeowners' insurance or renters' insurance. Just be clear what the categories are, and what sorts of ramifications you'd have.