Is willie alright with fish the same length as him. For alot of species if you put a new fish in that is 1/2 the size of a predator that means it is food to them.
fish will be fish... feeding first and a divider may help... he may even be ok with the new tankmate for a while... but hes still a fish... so if he ever decides he nolonger wants a roommate theres a good chance he wont go through the same eviction process people go through...