The difference with "fortification" infantry is the softness. Basically give them a hardness bonus for the fact that they dig into the terrain beyond what a normal "garrison" would do. Yes, you can build land/sea forts ... but the troops that the Japanese stationed on Pacific islands, and the ones the Germans had along the Channel, were active parts of the fortification process. If the HoI3 combat system makes attacking fortified/dugin troops tougher, though, this may not be needed at all. Maybe have a leader trait the opposite of "fortress buster" instead - give them a fortification bonus - I dunno. It's just kind of a let down sometimes when playing US how easy it is to roll up the Japanese-held islands in HoI2.
The difference between "light" inf and marines or mountain is the training time and organization. It's cheaper to churn out light-inf than specially trained troops, but they don't perform quite as well as "commando" type units either.
I agree, I don't think the light inf bde has to be represented by the more elite soldiers. In many suggestions in this thread, the light inf bde is based more around a difference in equipment rather than trg and experience. The mtn, marine and para bdes still have their places.