I'm not saying you couldn't do the killies in the 75. You could to the same number of pairs in there, heavily plant the tank though. They will generally not venture down below the mid line of the tank and spend most their time in the upper portions, so get some cories for the bottom, and maybe a pair of bristlenose plecos. By heavily planting the tank you may encourage the killies to venture down away from the top of the tank. That being said, if you only have those few killies, who's min tank size for a pair is about 2.5gals, the species I listed anyway, you could have a large school of cories and watch their natural behaviors. There's a lot of options, I'd just do some looking around and figure out what you want to do, you could probably get away with a couple pair's of rams in the 75 along with everything else, its really up to you. If you go with smaller fish and step into the planted tank realm you can come up with a work of art for a tank that size.