Jim,
Yes, the keel can be removable, in fact a lot of guys do it, makes it easier to ship it if you should need to transport it that way. I choose to have a fixed keel. With a removable keel you get the chance for water to enter the hull if it's not sealed well. I suppose if one had a removable keel, they could have several with different weights. But remember, the Soling 1M class has a minimum weight of 10#, and keep in mind, they say lighter is faster. As for plugs for the keel weight, that's not allowed. In this class the keel must be filled with #8 or #9 lead shot, and epoxy. As for weight distribution, about the only thing that can be moved, is the location of the battery pack for the receiver & servos. But in my case, my battery only weighs 4.3oz, not much to move around, and the weight does need to be as close to the center of the boat, and as low as possible.