There's no harm in asking for a specific outfit. We may or may not be able to accommodate, depending on the worker and what we have with us.
Be polite, but also to the point. We aren't able to answer every question someone has as it's time consuming, so narrow down what you want from the experience. For instance, asking if the provider does x, y, and z under the full service rate is a good question. Asking them to list everything they're willing to do for an hour is most likely not going to get a response.
It's natural to be nervous. I like to help put people at ease. Book enough time to chat a bit, or start with a massage, if that makes you feel more comfortable.