Not any cartridge will fit that filter your going to have to buy the Rena filter
as far as what to stock it with barbs and tetras with both give you the small school your looking for, check this site out you should be able to get some ideas here /
Aquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
I always cycled my tanks with goldfish but its up to you what you use you can use almost anything