K, well, it's fine if you have a main, but it's a good idea to have a secondary who can handle characters you're main usually can't. For example, if I were to main Ganondorf, he'd have a hard time against Fox, so i'd probably use Marth as a secondary. IMO, you're fine with Fox and Falcon as you're mains with your GaW as you're low tier main, since Fox can take care of Falco's, who would usually **** Falcon, and Falcon could take care of Marth's, who would usually **** Fox. That make sense? =P
Anyways, it's better to be good with a good group of characters that sort of "help" each other with their different strengths and stuff. So basically, practice mainly with Fox and Falcon, and you can sort of play around with Game and Watch.