You probably already know this, but just in case: Flat crt's will NOT work with lightguns. So keep that in mind.
My Trinitron is a flat-screen CRT, and my NES Zapper works fine with it. Where did you read this information from?
is it " crt is a crt" in terms of brand/ picture?
No. Like with most products where multiple companies take a crack at it, they vary in quality. Late-model Sony Trinitrons with YPbPr component inputs are said to be among the best consumer-grade CRTs you can get, while the PVM-BVM range is capable of RGB input via BNC cables along with other medical and broadcast monitors.
Flat screen CRTs tend to have a bend in the picture near the top of the screen. The bigger the TV is, the more noticeable the bend is. I have a Sony Trinitron KV-36FV26, and the bend is there, but it doesn't really bother me.
I've noticed this on one of my 20" flat CRT's.
Is this a geometry issue you can fix in the service menu or flyback pots (if you can get to either) ?
Adjusting it via the service menu depends on the brand of CRT you're using. With the Sony Wegas/late model Trinitrons using the special remote combination, you may try adjusting the VSIZ or VSCO, but I've heard you can damage your CRT this way if you force it too far out of spec.
As far as I know, adjusting the flyback pots messes around with the focus. I don't think it will fix the bend.
I've come to accept the bend as a natural flaw of CRT monitors. All good. At least we can bask in near-zero input lag and ghosting, natural 4:3 aspect ratio with overscan, and lightgun compatibility =)