Sure, you can do anything you want. There are no knitting police.
HOWEVER... Ribbing has a natural tendency to pull in which is why it's used on sweater bottoms, but with a side slit it will pull in and leave a gap. You might be able to experiment and maybe do a wider ribbing that doesn't pull in as much and/or use a larger needle.. :think:
BTW garter doesn't curl if you do it wide enough. Do at least an inch and it should be fine.