Keep in mind that many of the successful examples here involve heavily anodized aluminum. My Al spacers, for example, are heavily anodized black. Al seat posts these days are usually anodized black. The aluminum oxide coating on an anodized Al part passivates the surface to further anodic reaction with the carbon. Whether we are talking Al frame and carbon post, Al post and carbon frame, Al steerer and carbon spacers or Al spacers and carbon steerer, even if the Al surfaces are bare, there is no reason to not go ahead. Just pay attention. Keep the surfaces dry and especially free of salt water. Inspect the parts every now and then.