It isn’t great casting, but the others are questionable actors, or not actors at all. Leo has the chops to play the part, if not the age or look.
A lot can be done with makeup though, look what they did to Charlize Theron. Too soon to know if that will be enough. I wouldn’t say Billy Crudup looks particularly like Hoover, but he did a good job in Public Enemies. He is about the same age as Hoover was when that film took place though, looks like Leo will be about 6 years younger than Hoover was at the youngest the film will concentrate on.
Better casting if they just use her.