the standard rule for 1 goldfish= 10 gallons. is there one for tropical fish? for example, What is the rule for 1 angelfish? etc.

Standard rule for the tropical fish is

1 inch per fish per gal of water, so this means you get to have more, but you need to remember that we are talking about adult sized fish

i noticed in my tropical tank, my black tetras produces a lot less waste than goldfish. I have 2 of them in a 5 gallon tank, with a regular box filter. they seem to be happy, and both of them are only 2 inches in length.

For small tropicals (most tetras, danios, guppies, other small ones) it's 1 inch of ADULT fish per gallon of water, no matter what size they are when you buy them. For some fish, that rule doesn't work, because they might be territorial or like to swim really fast. Or they might be messy eaters, like piranha. Or with livebearers like guppies, if you have males and females, you might want a bigger tank because they might unexpectedly have babies and you don't want it throwing off your water quality. Sometimes you can have a little "extra" fish if you've got a type that helps clean up extra food on the bottom, like corys, in my opinion anyway. I'm not sure what angelfish are like, so do some reasearch, and if you're only keeping one or they're peaceful and not messy eaters, the 1 inch of adult size per gallon will probably work.