You could put in more goldfish. You could also try some White Cloud Mountain Minnows. If you want a Algae eater then you could get one or two Chinese Algae eaters.
The rule of thumb is usually 1 inch of fish per gallon. You could have 1 10 inch fish or 10 1 inch fish
it eats other fish
A 1 gallon tank could have one betta fish, but you will need to be diligent about changing the water on a regular basis. The smallest tank that should be used, even for a betta, is a 2.5 gallon tank as it at least gives the fish swimming room.
No it will get too big
Small community fish, 6 gallon tank is not much room for fish to move around once you include gravel, plants and other decorations, so you don't want any large fish. Platies, neon, black or glow-lite tetras. Standard rule of thumb is 1 inch of fish per gallon of water.
You could probably keep a school of a dozen Exodon Paradoxus in a 40 gallon, as long as they are kept well fed, with a riverine environment, and no other species of fish in the tank. Be prepared for a lot of work, but some enjoyable feeding sessions.
That all depends on how big the fish are. The rule is "1 inch of fish needs a minimum of 1 gallon of water". The tank could easilly be overcrowded if it is stocked with large cichlids and understocked (if there is such a thing) if the fish are Neon Tetras.
You cannot put a black kuhli loach in a 5 gallon tank at all, they need a 10 gallon tank at the very least. The maximum size that this loach gets is about 4-5 inches, so a 5 gallon is out of the question. As far as I have read, the black kuhli loaches like to be in schools of about 3-4, and you wouldn't want to have them alone, since the are community fish, they can go with platies, most tetra's, mollies, swordtail's, and small plecos. they just can't be alone or with fish bigger than them that will eat them. hope this answers you question.
length and width for a 55 gallon fish tank
Not totally sure but if the black Oscar is bigger that could be the reason
It is classed as a very small tank and could only house about 5 inches of fish. (1 goldfish is a 10 inch fish)
You could put the following: 3x Neon Tetra or... 1x Betta