"I'm currently taking Hydroxyzine at nighttime before bed for my allergy symptoms. Would it be okay/safe to smoke indica/indica-dominant strain before bedtime as well? Will the Hydroxyzine and cannabis react negatively with each other?"
The hydroxyzine may be potentiated by the Indica meaning your sedation level maybe much deeper than you expect. Why not consider dropping the hydroxyzine in about a month but start taking high-dose CBD with low THC for the day time to see if you can stop all inflammatory disease in your body including allergies. If you take CBD cannabis during the day, it is important to keep the THC level low so you can drive a car or go to work since you won't be 'high' at all. Then you can certainly take an Indica at night because you won't have to worry about a reaction. BHI makes a nasal spray with euphorbium and the more you use it, the less you need it and it stops allergies. That would also be a safe combination with cannabis.
I HAVE BEEN TAKEN HYDROXYZINE FOR MANY YEARS. I TAKE IT EVERY NIGHT FOR ITCHING. I VAPE INDICA EVERY NIGHT. I WAKE THE SAME TIME EVERY MORNING, NO PROPLEM. DO AN HONEST ASSESSMENT OF YOURSELF TAKING BOTH,GO FROM THERE.TREAT IT LIKE WHEN YOU SMOKE,VAPE. HOW LONG DOES IT TAKE TO FEEL THE WAY YOU WANT TO FEEL.