You can do just the one as long as you watch the fluid level in the master cylinder and don't allow air into it.
Since you shouldn't really keep and store brake fluid, I would flush as much as you can or are willing to. If the bleed nipples are really rusty...might be creating more work than you bargained for though.
One last thought...usually the seals go leaky because the edges of the cylinder or piston start to rust. If that is the case, stop and replace the whole thing.