10 gallon is the bare minimum *just* for the two black moors.
Not remotely. One moor needs 40 gallons when an adult, and a pair needs upwards of 50 gallons.
Upgrading graduallly can get expensive ( unless you source your tanks second hand ) and you will end up with a load of tanks you may not want or have space for.
It's far easier to save up and buy a ( second hand it's cheaper ) 55 gallon or bigger to begin with and save the hassle.
BETTA527 - A 10 gallon is no good for even one small goldfish of
any sort. It is however, good for a variety of small tropical fish.
If you still plan on getting a 5 gallon, then a Betta, 6 Endlers/guppies or a Honey gourami is a good choice for that size tank ( no fishy tankmates as it will overload the filter, shrimp are fine though ) .
As for the Huge goldfish pictured, his name is Bruce and he's approximately 17 inches long. Proof that goldfish should never be sold for tanks of under 40 gallons.
As an aside-
I personally think the sale of goldfish should be drastically reduced and
Gold barbs offered for tanks of 60- 80 litres - this seems to be what a lot of shops are recomending for goldfish tank size and while better than a bowl, it's still
far too small . Gold barbs are similar in shape and colour to normal bodied golds, are active, stay small and can be kept in a group of around 5 in a heated and filtered 60-80 litre tank.