The general concensus seems to be that a resident adult cat will more readily accept a kitten, as opposed to another adult cat, living in their territory.

It has been almost a month since Schuyler (white, almost 2yrs. old) has been co-mingling with my other two females and they still have an attitude toward him. I expected Lily (blue, elderly) would NEVER fully accept his presence, but I had hoped that Debra (blk./wht., about 2 yrs. old) would take to him and enjoy having a playmate. So far, it is not happening.

Best of luck with yours; it sounds like they are bonding!