First of all, the texture will replace the texture you have now, but you can change it at any time you want :) But unfortunately you can't have both textures to use selectively on canine or feline characters, the only way you can do that is to create a preset, using the fur texture you want to see on that character.
However, if you want to keep the texture you have for the canine, you do not need to use the canine fur texture you downloaded, you can just use the feline texture if you wish. It only needs replacing with the other feline fur textures in FeralHeart\\Media\\Textures c:
As for other people being able to see the same texture as you, they won't be able to. Only if they have the exact same ones, everyone uses fur textures differently, they aren't like Mass Markings. No one will see you as a 'marshmallow', they will just see you in the fur texture THEY have ^^
Hope this answers your questions!