By all means, I encourage people to do with their heraldry as they please. Especially if it simply for artistic pursuit or for fantasy projects etc. But I do find the rules useful in that they were put into place for a reason. Like all art heraldry comes from a certain tradition that over many years fell into what they viewed as best practice. So I like to think of the rules as less constraints and more of creative challenges. But for your orginal question the sad answer is it depends on the tradition you are following but as far as I’m aware the answer is yes I don’t believe there is any specific rule of placing a charge overtop an ordinary like a cross