Usually larger fish need more space, if you want a combination of medium sized fish and shoals, I'd recommend upgrading to a 50g or larger.
For a 20g tank, I would recommend only getting one shoal of fish as two shoals of six would require more space. Choose danios or tetras, preferably the smaller type and you can have about 6-8 as one shoal in that sized tank. Make sure to provide lots of hiding places for your fish as well as plants either plastic or live, although live is better to provide oxygen in your tank. Adding an air pump will help circulate the oxygen around the tank, adding a nice look and also making your fish healthier.
If you're not interested in installing a heater, you could have a shoal of white cloud minnows, they're very lively fish and would thrive as a shoal of 6-8 in your tank.
Good luck!