For crew battles, you're paying for 5 people in your crew to enter. Technically, you can put any member of the crew you want - you'll have to give us the list of names and characters from those who are in attendance. From there, feel free to mix and match. Think of it mainly as paying for 5 chances to win in a crew battle, and if you have more than 5, you can choose at any top who goes when.
However, if you pay for 5, then you need to play for 5. You can't remove one of those slots because it's what you registered for. The same when paying for 4 - you can't add another member in to make it 5 if that's not what you paid for. If you can't follow through with it, you'll have to forfeit.
That's a warning to you guys - if you paid for 4 or 5 man crews, you're going to play with either 4 people or 5 people that can participate. No exceptions.