welcome to fishlore!!!!!!!!
Allot of us have started out like you and the information given to us from fish stores, is awful most of the time
if your tank info is correct, you arent cycled yet as well as overstocked ...neither im sure is good to hear...the tank would be ok for your gourami, and maybe some pygmy cories..but that would be pushing it...smaller tanks are soo much harder to keep the water parimeters on track so you would have to keep up on water changes...the gourami would be more of a surface middle swimmer, and your pygmies would take up the bottom....but the tank has to finish being cycled...
if you are still using stress zyme, id stop using it and find some prime water conditioner..stress zyme doesnt contain the right bacteria and is eating up the good bacteria needed to finish up the cycle....the prime will help detox the
ammonia/nitrites for 24 hours until another
water change is needed...you need the daily water changes until you have 0 ammonia/nitrites and under 20 nitrates to keep the fish safe...
do you think you can find a 20g ? that would work for the fish you have and allow you to add a few more when your cycle is completed...check out
www.craigslist.org for a ton of great deals this time of year....
good luck!