Yes. But keep it to the lower dose if possible. Even better, if you are using it for allergies, the safest thing would be to try a prescription strength nasal spray for allergy symptom control.
Maybe. Allegra (fexofenadine) is metabolized mainly through the liver so the dosage will need to be adjusted. It will depend on how severe the liver disease and the dosage of allegra (fexofenadine). You need to discuss this with your doctor.