Adopting Stray: Stray is very agressive during introduction phase

Background:

I am hoping to adopt a stray/abandoned cat.

I took her to the vet got her fixed, tested and immunized.

During that time my resident cat was at my parents. I finally brought my cat home and she has the run of the upstairs while the new cat has run of the downstairs. (I live in a bi-level open loft, its the only way I could separate them.)

Anyways, new cat is EXTREMELY interested in what's upstairs and on the other side of the partitioned pet gate.

After a week of letting the cats hear and smell each other (smell swapping) treats galore near the partitioned pet gate, I thought I could test the waters with some visual contact.

I had TWO baby gates staggered at different sections of my staircase and the Extra tall pet gate up at the top.

I removed the partition and my resident cat went to the top landing next to the pet gate and peered down. New cat saw her, froze, stared and next thing I knew HURDLED over both baby gates (She looked like an Olympic style hurdle racer.) and was only thwarted by the extra tall pet gate.

I freaked out and my cat bolted under my bed.

I decided to put the partition back in front of the pet gate and am allowing just a small crack of site for visual contact.

I'm just wondering if stray outdoor cats are harder to introduce?

Why is this sweet little new cat so darn aggressive?

Is there any hope?

Sounds like she was under the impression that the house was hers and got quite a shock to see another cat.  Now that visual intros have been completed, you may want to continue the process.  Can you use the bathroom and put a barrier (stacked baby gates, a screen door) on the door?   Put the new cat in the room and the resident outside of it.  Switch the cats (resident in the room/new cat out of it) - say every other day.  This will give them each access to your whole place.

I know it's a pain but without a private room, this sounds like the only option.  The new cat sounds like quite the athlete.  I know you said she was angry.  Did she growl/scream or was she just hyper to meet your resident cat?

It sounds like every got a shake up.

The new cat was pregnant when I got her. I had a spay/terminate performed on her and she was recovering for a week at my condo waiting for her stitches to be removed.

As a result I had NO idea how athletic she was until now.

Each day it's kind of a new discovery as to how fast she can zoom around the living room, jump up to get a toy, or manuever the stairs. My mom calls he Macguyver. The vet said she was 3-5 y.o. But I don't know, this is one agile and spirited newbie.

The new cat and my cat have exchanged yowls and hisses in the past.

Todays Olympic hurdling contained no sound.

But newbies hairs were semi risen and tail mildly poofed upon rest. (I'd say half-power, I'd seen crazier puffy hair displays in the past.)

I've been avoiding putting my resident cat in the bathroom because a) its super small, like a water closet really, and b) she hates being confined. But it may be unavoidable.

The newbie cat sits outside the gate and puts her paw in the open crack and paws at the divider.

For the life of me I have NO idea what she'd do if she got in the bedroom with resident.

She could want to play....or it could turn into a blood bath. I think that's what scares me. I just don't know.

I may need to double stack the baby gates at some point. But thank you so much for the reply. It means a lot!
 

Ah, her hormones may not have calmed down yet, then.  Does your bedroom have a door?  Maybe you can use that room instead of the bathroom.

My sister had a similar problem with her dogs.  She ended up putting a kennel (yes, a kennel) in her living room to introduce everyone.  It worked, too!

Maybe you can do the same with a extra large dog crate?  Just make sure the cats take turns in it, so neither feels like the "prisoner."

Unfortunately my room does not have a door. Did I mention I live in a bi-level loft. All open concept. I didn't put the newbie upstairs because I figured at some point she could jump on the ledge of the retaining wall next to the stairs and get below. (My resident cat had her tail amputated; she's not a jumper).
The only doors in the whole place are to a broom closet, the bathroom and front and back doors.
I may have to try a dog kennel. That's a really helpful suggestion.