Mohannad, I appreciate you love your goldfish, but you have to realise that a 100 litre tank is not a permanent home for them. They will become stunted and could have health issues because of the over crowding in the tank.
So once you take the 4 sharks back to the pet store, you will have the goldfish to deal with.
Firstly you have mixed single tailed goldfish (the comets) with fancy goldfish (the moors and fantails), generally this isn't the recommended because as you can see, comets are more stream lined and faster moving than the moors/fantails. This can result in them out competing the fancy fish for food and may also cause stress by their quick darting movements, compared to the moors/fantails slower 'waddle' type swim.
Single tailed goldfish do much better in ponds, or very large aquaria, I imagine you have juveniles right now, but they can and will grow to 1 foot in length. Unless you can provide a pond or aquaria of at least 6ft x 3ft, I would suggest you look into re-homing both the comets. Perhaps someone near you has a pond? Or a local park or botanical garden has a pond that accepts goldfish? Just a quick note though; Do not release the goldfish into the wild, it's illegal to release fish were they aren't native, so don't go releasing them into any streams or rivers.
Myself, I currently have a single tailed goldfish and I'm in the process of organising a pond or another home for her. She is a juvenile, barely 3 1/2 inches and currently has a 90 litre/3 foot tank to herself! This isn't big enough for her as she will out grow it and I cannot add another fish, because it would over stock the tank. It demonstrates how much space these fish really need. It's hard to believe looking at her when she's so small, but remember fish do grow
Now this will leave you with the 6 fancy goldfish.
The general minimum rule for stocking fancy goldfish, is 20 gallons/80 litres for the first fish and 10 gallons/40 litres for each additional fish. This is the basic minimum requirement and will ensure the fish have plenty of space to grow and have room for exercise.
So for your 6 fish you would need a minimum tank size of 70 gallons which is roughly around 310 litres. Preferably one that is 3 or 4 foot in length, so they have lots of swimming room
The 100 litre tank will make do as a temporary
home, if you were to organise the larger tank for them
If that's not possible, then again, I'm afraid you would have to re-home them or take them back to the store. You could then always set up your 100 litre tank as a tropical set-up, there are lots of fish that could live happily in your tank permanently, it's just that sharks or goldfish aren't one of those.
Let us know how you get on and if you have any further questions. Best of luck!