I have 6 cats - all rescues - arrived at different times.
One littlle girl (1 year old) was very frightened on arrival and is taking a long time to rehabiliate. She's been with us 4 months so far.
She's been living in a room on her own and has gradually gained some trust in me. I let the kitten (6 months) in to play with her from time to time and that works mostly well.
The problem is that one of the other cats (1 year old too) has been very mean to timid cat. Dashing into her room whenever she gets the chance (which isn't often) and being mean and menancing - fighting, snarling, getting timid cat up a corner to frighten her some more, etc.
The plan is obviously, in the fullness of time, to integrate timid cat in with rest of the gang, but I don't know what to do about the aggression being shown by the other cat.