You've got a lot of questions in here.
Yes, get the 110 liter as soon as possible. (My converter says that's about 29 US gallons.) Those are the only two fish you can have in a tank that size. Repeat that to yourself every day. And even then, the 110 might not be big enough depending on what breed your calico is. (See below.)
When you get the 110, make sure to move your filter media from the old tank to the new one. When you got the 50 liter, unless you moved the filter media from the smaller tank, you started the cycling process all over again.
You need to be doing large daily water changes in a tank as small as 50 liters. Doing daily water changes in any tank where you are cycling with fish in is a good idea, and at a minimum, you must check your water chemistry each day to guide you in how often and how much water you change.
Get a test drop kit - don't mess with the test strips. API makes a good master test kit.
I know you must be dechlorinating your water during water changes, but with a tank as small as you have right now, ammonia, nitrites and nitrates can build up fast. It would be a good idea to get either some Amquel (made by Kordon) or Prime (made by Seachem) to use to detoxify the ammonia, nitrites and nitrates. These products protect the fish from specified levels of ammonia, but do not remove it, so it's still available to continue your cycle.
Goldfish tanks need to be double filtered, IMO. Getting one filter for a 110 liter tank is not enough filtration - get another one also. I have 3 filters on my 29 gallon goldfish tank - two HOBs, one rated for a 40 gallon tank, and one rated for a 30, and a sponge filter rated for a 40 gallon tank. (I run sponge filters on all my tanks, for the extra biological filtration, and in case of power failure, I can hook up battery operated air pumps to them.)
Even goldfish as small as yours are heavy waste producers. You said you got a calico goldfish, but you didn't say what breed. If it's a fantail, common, or comet, that fish needs to be rehomed as soon as possible to a pond, unless a tank of 55 US gallons or more is in your very near future. Your moor would be very comfortable by him/herself in your 110 liter tank. If you could post a photo of the calico, that would be helpful to other posters who come along to respond to your thread.
Surface agitation is important to keep sufficient oxygen available for goldfish. When you get your 110 liter, lower the water level to about 1/2 inch from the rim, so your filters can make a splash when the water from the outflow hits the surface. (I do not recommend that in your current 50 liter, because those fish need as much water as they can have available in that size tank.)
Now, about stunting. In the olden days, fish keepers were taught that fish grew into the size of their tanks. So small tank = small fish.
Now we know just how simplistic and incorrect that line of thinking is. Size of fish is reliant on multiple factors - water quality, quality and quantity of food, genetics. All these things combine with size of the environment to determine the ultimate adult size of the fish.
The reason goldfish in a well kept pond grow faster and larger is due to all the sorts of things I mentioned above. Goldfish in a pond can graze all day, which is the type of feeding they do in the wild. The algae in the pond, the roots of the floating pond plants, unlucky mosquitoes and other bugs who happen by - all this makes for a lovely snacking tray for the goldfish all day long.