I heard that you can go in and take pictures like for prom and not be charged. Is this correct?

You have to pass the entrance fee

Yes free

I did, nobody asked

Check their website under Plan Your Visit - Photography. Or call them.

Yes, it is allowed.

As long as you let them know what you are doing, yes. But it also never hurts to call ahead and speak with someone at the entrance so they have your info and know who to let in.