When I brought Weeble in, he was just a few days old, so I kept him away
from the others for his own safety - even though they were curious as to
what that noisy little creature that was getting all the attention was...

By the time he was old enough to 'get around on his own' (climb out of his
nest-box without help from me) the others had decided that they really
didn't want much to do with him, but that one shot of Tennessee hissing at
Weeble was taken when we had Weebs out on the bed for a bit of exercise,
Tennessee jumped up in an effort to get some attention too, noticed Weeble
and sniffed him (they actually touched noses), decided he didn't want nuttin
to do with him and hissed at him, then Tennessee fled the room.  Lizzie is
only about 1.5 years old, so she's still got enough kitten in her system to
find Weeble an interesting toy, but we kept an eye on their 'play' at first,
until we found out that Weeble was rougher with Lizzie than Lizzie was with
Weeble - she seems to know that Weeble is a kitten and when she 'attacks'
him, she does no harm (except to his pride), where as she now has some marks
on her nose where Weeble was a bit too rough with her, and bit/scratched
her, but he's still a kitten and doesn't know the difference between 'play'
fighting and 'real' fighting, but he's learning.

My suggestions for your new kitty is to keep her isolated in a separate room
for a while (we used a spare bedroom/office for Lizzie) until she shows
curiosity about the 'world outside'.  A 'cage is really no help for a cat,
they just want out! Once she shows interest, lock the existing cat up in
another room, and let new kitty out to explore for a few hours, then lock
her back in 'her' room and release the other cat.  After a while of doing
this EVERY day, letting the new kitty out for longer & longer periods, you
can start letting her out WITHOUT locking up your existing cat, since by now
they've gotten used to their scent.  But you need to supervise them!  Reward
your existing kitty with treats or hugs/kisses if he shows no hostility
toward new kitty, and same to the new kitty.  Since your new kitty is 5
years old, and was the only cat in her previous home, this may take weeks to
months and it's a pain, but it is the most 'stress free' way to introduce a
new cat, and she still may seek refuge in 'her' room from time to time
(Lizzie does, and we've had her for almost a year now).