Cardinals would be your best bet IMO. Congos can get a bit larger and aggressive like stated before. Do you only have 1 zebra loach and 1 GBR? If so, if you have a good amount of plant mass, you can add 20 cardinals IMO.
Personally I have a school of ~80 cardinals in my 95G in my signature and they school lovely, given that 35 of them are only half an inch and were just added yesterday, they're already schooling with the larger cardinals already. I've also noticed that they school better when they're the only fish in the tank. Originally I had 3 cherry barbs, 2 SAE's and 8 Roselines and they didn't school nearly as well as they do now.
I've heard the larger the shool, the better they look (especially with Cardinals). Yes, only one of each GBR and loach, was planning on getting another loach (LFS only had one left
) Wasn't expecting to get the cory cats but hubby brought some home as a surprise...*cough cough* which is why I have so many bottom dwellers. Not planning on another GBR though. Think I'll nix the Congos as they probably should have a minimum of a 4' long tank and so I don't have aggression issues. Thanks!