Bloodline limits are a genetic trait that lets you combine a strict 2 elements, not just any 2 elements of your choice. I'm not sure what Mizukage has, but she can only combine 2, whatever 2 they are.

I agree that the Tsuchikages probably got their ability from selective breeding and such.. though as for whether they can use them separately, I would think not. Not because there's any good reason why.. just to keep things special.