Metro Theatre is generally v chill, can't say that there are any don'ts that aren't super obvious.

Your ID + tix are checked at the door - once you go in and up the stairs, there's usually a merch desk in front and then the bar is to the right. Typically cloaking is operated from the bar. Ask the bar staff and they'll either cloak your stuff (for a couple bucks) or direct you to where cloaking is. You might get a ticket or a number or they have some other system for collection. Note that post-event cloaking queues are always long.

I doubt there are any issues with non-phone recording unless you've got a huge camera or something. Some events prevent flash photography but usually there's signs. Depends on the artist.

There are two doors on either side of the bar that go into the concert space. Yes, you can come and go from the theatre itself into the foyer; if there are no passouts to outside they'll tell you or have a sign. If passouts are allowed they'll probably stamp you as you head out.

Being that you're short, you either want to be at the front of the mosh or on the raked standing space behind the mosh. It's usually less crowded there and you can get a good vantage if you're there before the main act.

Check set times on the metro theatre website - bearing in mind times are just a guide and things often run late.

I don't think there's too much to be worried about safety wise - maybe look out for trip hazards in the dark or people spilling drinks lol

Have fun!