Mass roleclaim should, in all but endgame scenarios, result in the quick and efficient eradication of all power roles.
Mass nameclaim should never be a problem. If you have a source with a limited number of "gimme" characters, and more than about a third of the town consists of these characters, then your source is probably too narrow. Safe claims help. Not having alignment easily determinable from rolename, based on the source helps (see Harry Potter). The best approach, though, is to take a source with at least three times as many plasusible characters as there are players in the game, and then
don't
just cream off the 20 most obvious ones.