Uhh, I don't think so. You know, usually a main means that a specific character is the most suited for a player (be it because the player likes the character itself, or the gameplay it has) and/or is the one the player uses the most. At the end of the day there's going to be a character who is best suited to your play style, so that one is going to offer you a superior performance in a match than the others, making the idea of maining all of them a bit useless. Also you would have to use them all equally, otherwise like I said one or some of them will have more use than the others.
But anyway, if we get to the most direct definition of "main" and it's application, if you DO "main" all of them, like the first response said you're not really maining anyone. xD
I don't know, that is my honest opinion. But I get what you're saying, you want to use them all to a point of being good with all of them, and that, let me tell you is possible, but it's not the same as maining. I want to do something like that, I proposed myself to use most of them a lot to learn, because, well, if you know your opponent, you have an advantage. So yeah, that's what I think about all this. :D