Most churches in Rome have only slightly less rigid rules than the Vatican.

I ask guests not to smoke inside my house. There were two exceptions, and my brother is dead. Others are welcome to smoke outside. It's my house, after all, and I ask that you follow my rules.

Likewise the churches. Mine allows folks in wearing shorts, tee shirts, and other casual attire (near or complete nudity is forbidden). The Greek Orthodox two blocks away has different rules. The LDS churches have rules that would make your eyes bug out. You might not agree, you might think the rules are silly, but the place is theirs. Check first and if you don't like it, don't go.

As for flash photography: It can eventually damage fabrics and paper, such as books. Ask first, and a DSLR is preferable to a phone and you'll get a better picture. For interiors, most flash pictures are fairly useless if a phone is used because of distance other other factors -- get a DSLR.

We were in the cathedral in Toulouse and the inside was dark, dark, dark! Our friend Paula, a nun (wearing Bermuda-length shorts) was saying how her camera wouldn't get any pictures in that much dark. Suddenly the place LIT up! No, it wasn't a miracle -- a German television crew heard her problem and turned on the lights they were using to film the cathedral interior. Thus I highly recommend bringing your own television crew along.