Hi jcspr,
P&S does get a bit confusing with terminology like this, but here is my interpretation.
Light Cavalry are a troop type. Their key distinction is that they move 12” per move, rather than the 9” for other Cavalry.
Skirmisher is a ‘special rule’ that can be added as an attribute to a unit. Skirmishing is a formation, which has many bonuses / penalties described through the rules. Such units can change formation in to a Skirmish formation, and can change back. (My interpretation of p28 Changing Formation.)
As with most things I think you should be guided by how you think troops operated historically. For example you could decide that Croat cavalry in the TYW should always be in Skirmish formation and not allowed to change back and forth between ‘formed’ and skirmish. This certainly passes the “keep it simple” test.
Hope this helps.