Why do you want to know?
You can use anything you want, Letty. You can misspell every word and violate every accepted rule of grammar.
We are not talking about what you can or cannot do. We are talking about what grammarians normally refer to as, “correct grammar.”
The contradiction begins in the fact that the words "rule" and "grammar" have very different meanings to a scientist and to a layperson. The rules people learn (or more likely, fail to learn) in school are called [prescriptive] rules, prescribing how one "ought" to talk. Scientists studying language propose [descriptive] rules, describing how people [do] talk -- the way to determine whether a construction is "grammatical" is to find people who speak the language and ask them. Prescriptive and descriptive grammar are completely different things, and there is a good reason that scientists focus on the descriptive rules.
Most of the prescriptive rules of the language mavens make no sense on any level. They are bits of folklore that originated for screwball reasons several hundred years ago and have perpetuated themselves ever since. For as long as they have existed, speakers have flouted them, spawning identical plaints about the imminent decline of the language century after century. All the best writers in English have been among the flagrant flouters. The rules conform neither to logic nor tradition, and if they were ever followed they would force writers into fuzzy, clumsy, wordy, ambiguous, incomprehensible prose, in which certain thoughts are not expressible at all. Indeed, most of the "ignorant errors" these rules are supposed to correct display an elegant logic and an acute sensitivity to the grammatical texture of the language, to which the mavens are oblivious.
You can use either who or that to refer to people, JTT. Grammar is soooooo boring. Usage is another animal altogether.