you can keep them. The tv series Tanked did one for an ice bar pretty neat
You can, but I think they prefer really cold water. Also, I think that I read you have to provide looping water flow, because the jelly fish will get caught on the sides\corners of the tank.
from what i know jelly fish must be kept in a tank with no angles meaning it need sto be round . If not kept in one i think they will beat themselves to death in the corners.