First of all we need to clear something up here:
Dragoons were mounted infantry, they fought on foot.
Harquebusiers were mounted shot, they fought from horse back.
Dragoons, zipped about on a strategic level. For example, they would ride from point A to point B, normally places of significant importance like bridges/river crossings. The important thing to remember is that once they got to the battlefield, they dismounted. On the scale of a MED2 battlefield, they should always be dismounted.
Harquebusiers, were mounted shot, they travelled and fought on horseback. Originally they were used to screen and cover the advance of the heavy cavalry. But as cuirassier-style armour became more and more of a liability to the wearer, the harquebusier rapidly became the only cavalry on the field.
Essentially all cavalry in FKoC, except for the cuirassiers, are harquebusiers. However, the unit you are talking about is something of an acronystic unit. I wanted to show the origins of the unit type and decided to deck out that unit in an earlier (1610-1630) style.
I agree though, the Royalist harquebusier is a great tool, but don't let it get caught in melee!
Chees