1. Heck no! But you could probably do an awesome sorority and community tank. Betta included.
2. Yes you can divide it, using craft mesh and report binders is the usual way to divide.
3. It depends on who you talk to, some people say 5 gallons while others just say 1. And truthfully there are some fish who just /do not like/ big spaces and are more comfortable in smaller spaces. For me it would probably be 2.5 gallons.