Federer is actually quite under rated on clay.
He reached the quarter finals at Roland Garros 2001, and is still improving.
He can play on anything.
That said, he is more beatable on clay, than a faster surface.
For him it wouldn't have mattered what surface they picked - for his team mates, it makes more of a difference.
And as ys said - Kafel has won a slam on clay, and Marat grew up on it. They should be comfortable on it.