Is it a bad idea to introduce a new kitten to a house where there is already an older cat?

In recent days we have been giving home to a black kitten less than a month old yet to be named, the thing is that we already have a cat at home. My cat “Rubia” has been with us for 13 years now, beyond having lost her eyesight due to age, she is very healthy. But since the new kitty arrived I have seen her very stressed....