I don't think Bretonnian heads would fit right on the Paladins. Mantic minis just have a different scale, so the
GW heads would be far too large, even though the minis are the same height.
However, you can use Eccentric mini heads on Mantic Paladins to give them a Bretonnian look, although you'll have to add bling to the tops. Here is a picture of 2 Eccentric minis next to 2 Paladins: