Spas will never compromise your modesty. Most often, you will be given a robe to change into before your treatment. It's up to you. Therapists will only expose the portion of your body that is being treated at the time. If your treatment involves water therapy, a swimsuit may be appropriate.
Spas have different policies on tipping, but generally it is accepted.
It's important to ask about a spa's tipping policy at the front desk when you arrive. At the time of booking, express whether you prefer a male of female therapist. Many spas will ask this question when booking your service. They're there for you and their goal is to provide you with your perfect spa experience. There's no pressure to talk during a treatment.