Sam,
That tank is too small for goldfish. ALL goldfish are messy as all get out and can grow from 6-8".
Now, do I think you could handle a Ryukin? Yes and no
Yes, IF you follow peoples' recommendations here. No, if you won't or don't upgrade 
1. Okay, first of all you need to decide if you want to keep those two you have or not. I don't really *know* what they are. They look like comets, but, as you said, they are French and you guys might have different categories for your goldfish. Dunno. Find out what kind of goldfish those are. I'd surf some French sites and see if you can get a positive ID.
2. If you decide to keep the two you have: they need to be upgraded to a bigger tank. They will need 45 gallons with a 90gallon filter.
3. Do NOT buy a Ryukin or any other Fancy and put them in with those two. Those look like Group 1 goldies and they will harass Group 2 and Group 3 fish to death. Ryukins are Group 2 fish.
4. Get a 30 gallon tank for ONE Ryukin. Get a filter rated up to 60+ gallons. You don't need a thermometer. If you want more than one fish, add 15 gallons onto the 30 for each fish. Double the gallons (or litres) to find out what filtering you need.
5. Buy test liquid kits. Cycle the tank. Read the sticky in the Freshwater Newbie forum on Cycling
6. Buy your Ryukin
Goldfish are great for beginners, but only if they are housed properly.
Hope this helps
Roan
That tank is too small for goldfish. ALL goldfish are messy as all get out and can grow from 6-8".
Now, do I think you could handle a Ryukin? Yes and no
1. Okay, first of all you need to decide if you want to keep those two you have or not. I don't really *know* what they are. They look like comets, but, as you said, they are French and you guys might have different categories for your goldfish. Dunno. Find out what kind of goldfish those are. I'd surf some French sites and see if you can get a positive ID.
2. If you decide to keep the two you have: they need to be upgraded to a bigger tank. They will need 45 gallons with a 90gallon filter.
3. Do NOT buy a Ryukin or any other Fancy and put them in with those two. Those look like Group 1 goldies and they will harass Group 2 and Group 3 fish to death. Ryukins are Group 2 fish.
4. Get a 30 gallon tank for ONE Ryukin. Get a filter rated up to 60+ gallons. You don't need a thermometer. If you want more than one fish, add 15 gallons onto the 30 for each fish. Double the gallons (or litres) to find out what filtering you need.
5. Buy test liquid kits. Cycle the tank. Read the sticky in the Freshwater Newbie forum on Cycling
6. Buy your Ryukin
Goldfish are great for beginners, but only if they are housed properly.
Hope this helps
Roan