2Wire modems almost always work fine for Fusion. The splitters that they come with though cause nothing but problems so if you are using one, you should probably remove it. Also, the filters it comes with are not ADSL2+ compatible so you may have more noise on your line than if you get ADSL2+ filters. Some people have no trouble using the older filters while some do.
About your AT&T DSL, as soon as your Fusion service is active, your account with AT&T will be canceled. The phone and the DSL portion usually activate about the same time (the service itself takes 3-7 business days). It never hurts to call them and make sure that they removed you from their billing. You will lose connectivity though because your 2Wire will require a configuration change to work with Fusion. If you are familiar with logging into the 2Wire and changing the settings, the only thing you need to do is change from PPPoE to Direct IP (DHCP or Static).