Yes it’s perfectly fine to register gambling domain names at Godaddy.
You can go down either route when it comes to question 1. Some people advise starting out focusing a mini site on just one online casino / poker room and others like to build a bigger more general type site which has space for content.
If your skills lie strongly in link building I’d go for the mini site first but If you’re more of a writer and long tail guy then make sue you use these skills.