How many fish total can I put in a 15 or 20 gallon aquarium?

There has never been an inch per gallon rule its a myth.

7 Fancy Guppies

7 Neon Tetra

5 Danio

"I usually go by the one gallon per one inch of fish (grown). Does this still apply?", No, it is actually a myth, so don't use it even for grown fish.

3 fancy guppies - ensure that they are ALL male, otherwise you will have an overstocked tank.

5 neons

3 zebra danio.

Try an algae eater too, like a nerita snail or bristlenose.

1 fish per 10 gallons

The one inch per gallon is only a guideline it doesn't take into account whether its a slender fish like tetras, angels silver dollars or a fish broad fish like catfish, puffers, guppies or a tall fish like again angels or discus, i use a 10 gallon for 4guppies and their fry and in my 20 gallon i have 6hengrls rasbora, am gettimg 3otocinclus catfish amd have one juvenile ram which I'm getting 3more but will be getting a 50 gallon before they are full grown in the next couple month's but that's it for stock

depend on how big ur fish. normally 1 inch goldfish per 3 gallons water. You may ask pet owner for an advise.

About 30 cod fish