Can I use a 1.5mm tip with this?

I'll throw my .02 in the ring - though I'm a little reluctant since anyone who's done any painting thinks they're right and everyone else's way is wrong. I don't claim to be an expert, but do speak from my own experience.

For starters, I think you'll be fine with a 1.5 tip.

As far as the Flatz go or other "Hot Rod Black" finishes, they can be tricky to do without ending with splotchiness here or there (and I'm not talking about dry edges). Even the pros I have talked to get mixed results sometimes. Some of it has to do with temperature and reducers. It's gotta go on wet and stay wet for a bit. From my oberservation, if the hood has begun to dry in places before you finish, there's a good chance it will have some blotchiness when it dries.

I define blotchiness as areas that appear a little more glossy or flatter than others.

Also, remember, if you spray it laying flat and any trash gets in it, you'll either have to live with the trash or sand and redo it. However, if you hang the hood vertically you won't have the horizontal surface area to contend with - though you will have to worry more about runs.