Hmm, mixed units is actually possible now I think.
If you dismount dragoons their horses can be shot separately, then when you re-mount if there are more soldiers than horses you get a few guys following behind on foot.

Dragoons are nothing special as infantry so I don't dismount them often but I always found it an interesting trade-off whether or not to dismount Chivalric Knights in Medieval; they could provide an excellent anchor for weak parts of the line or spearhead an assault to break into a castle etc, but then they were very good & expensive cavalry so the loss of the mobile force could be more important than the infantry strength gained.