If they were doing well without grain on the 20 acres, they shouldn't need it on 6 either. Will they be out on all 6 acres all the time, or are you splitting into sections to rotate pastures, putting them in a sacrifice area at night, etc.?
You'll probably need to supplement hay (have it available free choice if possible, at least for the nursing mare), and adding in a ration balancer wouldn't be a bad idea either. A specific mare & foal formula might be a good idea for the mom and baby.
I'd definitely avoid sweet feed- horses don't handle sugar and starch very well, and sweet feeds are high in both of those.