Yes - waivers do occur. Yes your rank can be influenced by specialty training you have had as well as how long you have practiced. Without any Post grad training - you may only come in as a Captain. Realize that there will be some dentists younger than you that may be your supervisor.
Best thing to do is talk to the recruiter of the branch/organization you are interested in - Navy, AF, Army, Reserve, Guard - they will have the specifics for what is required.