Agencies have rules against contacting their ladies directly because they don't want their ladies doing any side business.  

It is not tacky to asked a lady for a reference after your first date. You are still new and trying to establish yourself as a credible hobbyist.

It is not considered tacky but some agency girls are not willing to do anything outside of the agency including reviews. Depends on the girl.

...In the case of an agency girl though you probably won't be able to for the reasons stated by a previous poster. The agency may give you a reference. Some agencies do some don't. Independent providers may not accept a reference from an agency. Some will some won't.

you don't have much of a choice, eh?
Where else are you gonna get a reference for your second session with someone? No harm in asking.