Kittens are tolerated by older cats. If your going to keep this kitten don't just put them together immediately. You will have hissing, swatting and possible biting. This kitten is coming into the established cats territory. Keep them inseparate rooms. Let them see each other thru cracking the door. Depending on the reaction, then you can let the kitten out under supervised interaction. I've had a bunch of foster kittens come thru my home. That is how I did introductions. Make sure you get this kitten spayed/neutered soon. Rule of thumb here is once they are 3 pounds. Your so kind to rescue this kitten. Im sure it will make a great companion for your 11 month old cat.