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.