Meh.. don't keep the clown in that 10G. If you have a bigger tank, put him in there. Keeping him in the 10G IMO
, is unfair. He may be able to survive, but not as happily as he would in a larger tank. Their tank minimums are 20G.. so.. yeah. I said it before; A child could survive in a closet with food and water.. but how happy would he/she be.. living out their lives in a closet. Putting a clown in a 10G is like putting a naso tang in a 90G. Its big for sure.. but not big enough, so that the naso can fully grow and live a healthy life.
As for the whole 10G QT thing.. pass on the 2-3 inches per fish thing. It soley depends on what type, and how many fish are going in there. Put a full grown naso tang in a 10G QT tank.. and see how that goes. Your QT tank needs to be as relevant as your fish are. Small fish, small QT tank is fine. Large fish, with high bioloads, large QT tank with more water to dilute as best as possible the ammonia and waste they produce.