new baby in the house

I am now the proud owner of a 4-month old baby female kitten (Sneetches) and
I already have 28 other cats. A 12-yr old male Himalayan (Petey) and a 4-yr
old female calico (Sox). Here's the background story on my 2 oldest. I was
there for the birth of Sox. Was so beautiful! So, I have had her since day
one and she thinks of me as her mother. Petey was adopted 2 years ago when
he was 10. I was working at PetSmart and it broke my heart seeing him
there. He's my big mush and loves any lap he sees.
Two days ago a friend of mine called. She has been trapping feral cats
where she lives, getting them spayed etc.. before releasing them back into
the wild. She calls and told me that she trapped this male cat who had this
tiny female kitten in tow. She had them both spayed and realized that the
female wanted her attention. Sensing the female could be easily socialized
she started the process and also started hitting her friends up for a home
for the little one. That's when she called me. I of course said yes and
now have 3 cats.
Sneetches is a little mush. She won't leave my lap. She will be spending a
week or two in a pet container in my living room. She will be coming out
periodically for attention and socialization. At this very moment she is
sitting in my lap, with her head on my hand, purring up a storm. She wants
to be out of her cage more but I won't let her yet.
This is where I need some help and advice. When I brought Petey into my
home it only took a few days for him and Sox to get along. Sox was used to
going back and forth to visit her brothers so another cat wasn't problem.
Also, right after adopting Petey we moved into our house. The problem I
need help with is this. How long will it take my 2 older cats to stop
hissing at the new baby and at me? When will I be able to let my new baby
wander around the house? Is there anything I should do to help make this
situation all better for the 2 of them?
Thanks in advance for any and all advice.

Jenni