1 fish inch per gallon is a useful rule of thumb. You've probably got twice the amount of fish in there than you should have already, and that's assuming they are all fully grown.
My advice is that to be nice to your fish, certainly don't get any more, and consider reducing your stock or getting a bigger tank.
We're agreed on that, regardless of anything else.
Didn't spot the "guppy tank" bit. He did say 6 fish under 4 inches long though. I know that an inch or so is technically under 4 inches, but I'm assuming they are 4" and aren't guppies.
Unless he lives by a nuclear power station and rescued them from the wild.