Welcome! What a wonderful person you are for taking that poor baby in and rescuing her!

When we got Pouncer last year, the beginning brought a bit of hissing from Allen and a few playful fights (I dunno... Allen did look mighty serious though) First thing you need to do is just keep them separated a few days. Just so the kitten gets accustomed to her new home and your other cat can get used to Marley's smell on you. Then slowly introduce them together. We let them spend a few mintues together and pulled them apart after the initial sniffing. This gave Allen enough time to realize a new critter was in HIS space. Eventually, Pouncer and Allen were allowed to spend free tiem together. Even then there were fights. Mostly because Pouncer wanted to get up into ALLEN'S chair. That was his throne - NOBODY went there... but Pouncer did. So fights broke out over the chair. But now the boys absolutely LOVE each other.

I hope you have a room that you can put Marley for a few days. Pouncer was caged in my daughter's room. A lot of people use a spare bedroom or second bathroom. It does make a big difference on how well the other cats accept the new ones.