want to add more fish

Im a beginner, have a 20 gallon tank which cycled for 5 wks (fishless cycle) gravel, fake plants, whisper air pump and Aqua clear filter rated for up to 40 gallons. Added 2 zebra and 1 leopard danio at first, after 2 wks added 3 rosy danios, another 2 wks added 5 dwarf corydoras.
So can i add more fish, i know about the "the rule of thumb" but with adequate filtration, proper maintenance and regular partial water changes.

Ammonia - 0
Nitrite - 0
Nitrate - 5
pH - 7.2
Temp - 24 deg Celsius
Use Nutrafin liquid test kit. I do weekly 25% water change.
Have not had any casualties since introducing fish. Tank has been running for 2 months now.

By the way thanks guys used this site as my main source of information for starting fishkeeping.