Hi Gogecko
What rudders have you got now? are they the kick up type with the fibreglass rod?
How shallow is shallow? I don't think I'd want to sail a A cat with a foot or so clearance, a hobie 16 or something i'd be more comfortable. Just sounds expensive with a A class.
Shorter boards and rudders are going to effect the amount of control you have, harder to steer, be a bugger of a thing to tack, won't point etc...
But I suppose if your just cruising around you could try it out, make them a bit wider and shorter, how short? that depends on what your dealing with I guess.
Regards