I want the wedding to be as informal as possible, since it is a second marriage for both. I don't want a head table and I don't want people to feel restricted to where they have to sit. Is that okay?

Sure. You can structure your ceremony and reception however you like it. It can certainly be informal seating. You may want to have special centerpieces or decorations or reserved signs for the bridal party tables if they are all going to sit together, otherwise the is no reason your guests can't just sit where they want.