Hi,
The short answer to both question is yes.
Sex change occurs in captivity. Clownfish are protandrous (male first), thus the fish begins its life as a young adolescent male (I do not agree with the poster that Nemo could not have existed.).
The female in clownfish is the most aggressive fish (this fact was well shown in Nemo), and she will harrass the male all the time.
Once a female is killed or dies from natural causes, the biggest male in the spawning group takes over and becomes female. (This was NOT SHOWN IN THE MOVIE!!! Marlin would have been MOM not DAD !!!

) The next fish in the pecking order becomes a functional male...
Clownfish do not require an anemone to thrive in captivity, but they will usually choose something in the tank (a rock, an algae turf, etc) as a surrogate host. To be happy, clownfish need somewhere to seek cover.... but an anemone is not necesarry.
The clownfish-host anemone relationship is exhibited in captivity and they may choose a species of anemone different from the one they are usually associated in nature...