IME you will not see much of the CPDs if you keep them with larger schoolers like the harlequins. I've tried it in a 45 gallon heavily planted tank with furcata rainbows, cardinal tetras, and rummynose tetras, and they were so shy that the other fish outcompeted them for food and the CPDs slowly died off.
CPDs are better suited to smaller tanks IMO, based on my experience with them. I couldn't keep them alive in a larger tank, but I've bred them in a 10 gallon. If you want to try CPDs (or Boraras, or some of the Microrasboras) I'd suggest a small tank with plenty of plant cover. They'll show a lot more color and activity. I personally would not go larger than 20 gallons with a group of CPDs.