Stocking a 20 gallon tank

Question
Hi,
I have a 20 gallon high aquarium. It has blue gravel, 4 small plastic plants, a medium-sized rock cave, and a small mountain thing that fish can hide in. The filter is an AquaClear 30, there is a 100 watt heater, and the temperature is currently 76 degrees.
The tank seems to be cycled. When I last checked, the ammonia and nitrates were at 0 and the nitrates were at 10 ppm. The aquarium has been set up for almost a year.
For fish there are:
4 zebra danios (2 are long-finned, and are a bit larger than the other two).
2 male guppies
1 otocinclus catfish
All of these fish have been in the tank for at least 3 months.
How much more space do I have for other fish? Do any of my current fish need more friends of the same species? I like dwarf gouramis, honey gouramis, and platies, and am trying to figure out what else I could put in the tank. I realize a 20 gallon long aquarium would be better, but I didn't know that when I bought the tank.

Thanks,
Kate

Answer
Hi Kate,
You mentioned nitrAtes twice.  You probably meant "ammonia and nitrItes".  And yes, it does sound like your tank is cycled, good work :).  

You have a lot of space left if you are considering these small size fish.  There is a "guideline" that says one inch of fish per gallon.  But it doesn't always work very well due to different sizes of fish.  I would suggest in your 20gallon, you can keep up to 20-30 fish if they remain that small.  I also have a 20 gallon tank but it is long, not high, and I have about 50 platies ranging from babies to adults, if they were all adults I would be overstocking.  You can visualize if it is overstocked or not.  If it doesn't seem like they have room to swim, bumping into each other and etc, then you need more space.  If you decide in the future to get a long tank, you can stock more.

All of the fish you currently have, the danio, guppies, and oto cats, are all schooling fish and need 5 or more of its kind to be happy.  

Dwarf Gouramis get twice as big as the fish you currently have and will need more space which you will have to lower your fish population if you get them.  Also, all Gouramis are known to be semi-aggressive and might try to chase/fight some of the other fish.  I recommend keeping the same temperament types of fish together.

The platies will go perfectly fine as these fish are very peaceful and are non aggressive.

Hope this helps!
-Matt-