Can I call an assistant professor "Professor"?
Yes, you can call any kind of professor a professor, and you should.  Addressing someone as "assistant professor" or "associate professor" would be...well, it's simply not done, so I can't say if it would be rude or just weird.

It is similar to military protocol, actually: e.g. if someone is a rear or vice admiral, you call them admiral.  In contrast to the above example, I'm sure some knowledgeable party could be specific about what bad thing could happen to you if you screw that up!