The main issue is tank size, and you don't indicate how large your tank is. But as the previously-linked article (that is an excellent site and very trustworthy by the way) points out, they must be in a group (loaches are very social fish) and given its size the minimum tank size is 75g when they are small. Eventually they will need larger tanks to be healthy.
Keeping social fish singly causes stress and that means poor health and more likelihood of disease. Keeping potentially large fish in small quarters even when young is also detrimental because it causes internal problems that will later result in poor health and probably shorter lives than normal. In other words, deformity.
If your tank is 75+ gallons, a few more clowns will be good. If the tank is smaller, I would see if the store will exchange the clown for another species. There are several loaches that remain smaller, up to four inches maximum, and these would be better provided the tank is adequate in size. In another thread you mention a betta that killed one loach and is now after the remaining one; another issue, but if this is a small tank I would not recommend loaches with the betta.