I have both. I've had bad batches of both, with huge attrition. My most recent batch of Cardinals--small ones (now grown)--had 0 losses.
Make sure the shop you buy from has had them a while. It makes all the difference.
As far as schooling behavior--that's a great question. Either will school well out of the bag. I have a 26-G aquarium that is 3 ft. long. When I had 4 Cardinals after a big loss, they stuck together well. When I added more (now 15), they often break apart in 2 sub groups. I still have 3 Harlequins and they stick together quite well, although 2 posture a lot to the exclusion of the other.
Soooooo ... I suggest buying about 6 of one or the other for a 26-G. Too many without enough room, and they won't school. Too few, and they won't thrive.
Re: shrimps--I have never seen a Card or a HR go after my juvenile or adult Cherry Shrimp. The shrimp are reproducing without incident.