Is it okay to ask a professor what they expect in a grad student for a research assistantship position?
You should definitely ask him. But at the same time let him know that you have gone through his course details online. In your conversation with him ensure that you are not asking anything which is readily available on the university website. In other words, you should reflect in your actions that you are sincere about your queries and not wasting his time.
You should definitely ask the previous students. You can glean from their experiences to find out if this position is suitable for you.