If you want Malawi mbunas then they'll be the only fish in the tank. You need masses of rockwork to give them hiding places and would be looking at around 20-30 fish in the 260 or 30-40 in the 450 depending on which species you want to keep.
I agree, looks around 80 or so which may well be the cause of the problem. Test your tap water nitrate level too as if it's high (mine is) it will be much harder to get the tank down to a suitable level and you will need to consider other options for doing so such as the use of RO water or nitrate absorbing filtration media.
What's your nitrate reading? Sometimes, particularly as they age, goldies can have swim bladder issues triggered by nitrate in the water and you need to keep it pretty low for them (ideally under 20ppm).
However, 30 litre is extremely small for a goldfish and he looks stunted - at 2 years old he should be a very large, 8-inch long fish (excluding tail). He is most likely suffering from poisoning from excess nitrate due to the tank being too small and unfortunately when they get to the stage of bending into a C shape like that there is often very little chance of recovery. You can try though by changing most of his tank water - BUT in small stages, a bit at a time over a couple of days and make sure to use a dechlorinator on the new water.
If he recovers, make sure when you put him in his new tank that you move his existing filter over too or he will be at risk of ammonia and nitrite poisoning as the new tank and filter won't have the beneficial bacteria that convert his waste into less harmful nitrate (which you will control by doing a regular weekly partial water change of at least 25%)..
Neither is easier; the only real difference is you need a heater for tropicals. Tropicals give you far more choice though as there are very few small cold water fish that are suitable for a tank of that size (for instance it's much too small for goldfish). You don't need an air pump as long as you have a good filter with its return disturbing the surface. Live plants aren't essential.
The nitrite reading is dangerously high. You need to change a lot of water to get that down to 0. Use a conditioner called Pond Prime in a double dose which will protect the fish from the effects of the nitrite for 24 hours between water changes.