@Tania23 I don't see why not so long as they don't conflict with Airbnb's Terms of Service. A good place to put them is in your House Rules. If you require a signed contract, you must disclosed it in your listing and in communication with the guest prior to accepting a reservation.
If you can, I'd turn off instant book since you'll need to have some communication with guests. If you can't, I'd contact Airbnb and explaing that IB really isn't suitable since you will need guests to agree to a contract / Terms before booking. Click on the link below to see all the ways to get in touch with Airbnb.