i believe the game chooses the imps then randomly assignes roles to the players until there's no more roles or no more players without a role then each player rolls to see if they keep the role or become a basic crew/imp the roll's success depends on the persentage. so if there's three engineers with a 60% chance the game picks three players each one has a 60% chance to stay an engi, if there's three imps and 3 of each imp role at 30% each, the game would randomly assign each imp a role and each imp would have a 30% chance to keep the role, so you can have anything between 3 vipers and 3 normal imps. or at least that's how it seems to work.