I don't know how to do exactly what you're asking, but here's my own workaround.
I don't know how to combine the buddy list windows, but I have a widget in my dashboard that shows my buddies online, and it combines both the AIM and Jabber buddies into one list. You can chat with people directly from the widget by clicking on their name. I just close my buddy chat windows and then use F12 to bring up the dashboard when I want to chat/check my buddy list.
Re: checking the gmail, I don't think ichat will do it for you but I have set up my gmail in macmail. The instructions are in your gmail help. I also changed the preferences in macmail to check for new messages every one minute (as opposed to the default of every 10 minutes), so when a new message comes I get it quickly. When a new message comes in it makes a chime and shows a little red dot on the Mail icon in my dock.
Chax will do this, along with a lot of other little iChat improvements. iChat s really not complete without it. You will still be able to open up individual buddy lists, but there will also be an "All Contacts" list available.
As for e-mail, I don't know why you'd ever expect it to do that, it's a chat app. But you can use Mail.app to get your mail from Gmail, or if you prefer the web interface and just want to be notified of new mail, use Gmail Notifier or Google Notifier or whatever it's called.
But anyway, Chax comes extremely highly recommended. You just install the plugin and then it's so seamless you forget it's there.