They are fine at the moment debs but will need room to grow. They will be fine for a year or two yet
Personally I do not like to see comets in aquariums, they should be out in nice big ponds. The fancy goldfishes with the long tails are different as they do not swim far or fast.
As a rough guide, to work out how many coldwater fish you can keep, multiply the length and breadth and divide the result by 76 to give you the total amount of fish you can keep in cms. So, 50 x 25 divided by 76 = 16.4cm total. So, theoretically, this means 1 fish 16.4cm or 2 fish 8.2cm or 3 fish 5.4cm etc.
This is a rough guide only as the bigger the fish, the more waste it produces. 3 small 3cm fish will not produce anywhere near the same amount of waste as one 9cm fish. Especially goldfish as they are a full bodied fish and poo heaps!
Good filtration and/or regular water changes are a must for goldfish to be well cared for. Avoid undergravel filters with goldfish as they dig through the substrate and have a tendency to expose the filter plates with their digging.