One can install breakers/fuses smaller in ampacity than the rated enclosure they are installed into, as long as the breaker/fuse is listed ('fits') for the enclosure.
So a 40 amp breaker/fuse can be installed in a 60 amp rated enclosure fed by 8 AWG. See Art 240 and Art 408.
What some may want to do (possibly exceeding code requirements) is up to the individual, but code is minimum and an inspector cannot ask for anything more.
Some of us have installed work a certain way as a practice, and when we see a 'code' minimum installation we may cringe, but it is permitted none the less.
Pierre
