If the Acara isn't so juvenile to be small enough to be eaten by the giant danios it isn't a big problem. Just keep in mind Cichlids have hormones they excrete into the water that can reduce their own growth if not removed via water changes. This is easier to keep up with in larger quarters, so even if the size now is nitrogen cycle ok in the smaller tank, it may not be ok due to growth inhibiting hormones building up faster in smaller spaces.
Plan on changing as much volume of water in the small tank as you would for regular maintenance on the larger tank. If that amount is more than the smaller tank holds divide the amount and do two or more changes a week. This should ensure that the smaller tank does not stunt the growth of the Acara.