Well, most of your questions can be found in a early Roman History book. As to the questions of what makes a tribe, I think it's possible to generalize that when referring to the Italian tribes, it generally referred to a group of thousands which lived in a general area and were generally related by blood. Lots of generalities I know, but that's the fact, all borders are porus.
When you refer to the Siculi and Sicani, I'm fairly certain they lived in Sicilia and not southern Italia. I bet Panthagus would be great for these questions.