Thank you for adopting a new feline that needs a home.
You're introducing a new cat into the others' domain. Cats are very territorial. It will take some time for them to acclimate to each other.
The younger cat is just wanting to play. The older cats are probably a little past that stage in their development. At 8 and 9 years of age they are pretty much in their senior years! That's how quickly these animals age. Just give it some time and see how it pans out. It's only been just over a week, right?
They may even end up playing with each other, which will keep your geriatric cats youthful and it will force them to get some much needed exercise. When I introduced a younger female cat into my 10 year old male cat's domain, she was very inquisitive and he hid underneath the bed for 3 whole days! Now that's bladder control! :eek:
As far as them hissing around, cats spend a lot of time posturing but they are seldom aggressive animals. You may notice this mostly around meal time. It would still be a good idea to keep them separated during the day while you're at work or away.