Very nice looking Ceylon, should do very well for you, they are great fish. I agree with the advice that you are being given. They are also one of the relatively few puffers that look better with maturity and age - some look best and brightest young, but Ceylons keep improving.
The low nitrate reading indicates that your water changes are enough for now while the fish is immature, but you will need to boost the percentage changed or the frequency fo changes as the fish grows - bigger fish eat more and poop more, and puffers are messy eaters at best - so there will be more waste to be removed. Right now you are managing the tank well.
...Tears
