Darryl the position of the foils on the rudder is more critical with your setup as you need to be able to lift the blades clear of the transome.
Another problem might occure when raising 'kick up' rudders. The lower the foils are, the more drag they would cause as they are being raised, effectively neutralising steering. This would make it very difficult to sail in shallower waters where the rudders can't be locked down untill the water is deap enough. In that case they'd need to be up as high as is practical. A long-ish blade would also help.