There are no rules except the ones you make. It's fantasy, it's fiction. That being said if you really, really want to look in-depth at it (and I say all this having classified my own world as steampunk) there's a book you can get that's a good reference and jumping off point. I do like my reference books they're half my collection. You never know when you'll need 'How to Read Linear B' or 'The Forensic Anthropology Fieldbook'. (And yes, both have come in handy.)