Note that the 'C' refits don't always follow these rules- the Champion and Battlemaster both got 'C' refits that end up being pretty awesome Mechs (the Battlemaster-C may be one of the games best assault Mechs in fact), but have toys like double heat sinks, Clan internal components, and the like. Whereas the Warhammer, Victor, etc. were refitted isorla Mechs, the Battlemaster is even new construction (not sure on the Champion). And yet, still a 'C' design, not a 'IIC' design. Oh, and then there's the Marauder II-C, the Clan-tech version of the Marauder II that the Dragoons used to use, totally blowing any semblance of rules on building Clan refit Mechs out of the water.
Welcome to a world without rules. ;)