To properly represent them as dismounted, there should be two mounted squadrons. I usually field the mounted squadron and use the foot troops as commanded shot.
Finally for Waller's army, I have two cannons and a stand of artillery train guard. In Victory Without Quarter (my free rules in case readers don't know), you can have an artillery train guard deployed with your guns. They give some small measure of protection with the ability to stand and fire at chargers - not a huge defense, but better than nothing!
These models will work as easily for Royalists as Roundheads (as will most ECW units with a quick flag swap)!