I know that Feral cats get caught to be spayed or neutered and then they tip the ear so that people know that the cat has been fixed. They generally return them to the “wild” after getting tipped. You may have just adopted a feral! Check out Kitten Lady on Instagram. She is an expert cat rescuer and talks all about it.
Brett and Beth, Thank you for caring for her. Couple things–if you can get her into a carrier and to the vet, they can scan her for a microchip. The vet can also have Seattle Animal Control come out and pick her up or you can take her there.
With an ear tip, comes a rabies vaccine.
There is good information on introducing a new cat to the household on the PAWS website. Key is separating the cats for at least a week, with the new cat being in a small room such as a bathroom, office, or extra bedroom. Calicos are notoriously opinionated, so you may have your work cut out for you.