How do properly introduce a tamed cat to 2 feral cats?

[Nichita]

I had to place about a year ago two feral cats (Mom and 6 months baby) in an 11' X 12' enclosure room. The family consisted of Mom and 3 kittens, they were trapped, spayed/neuter and released, but my neighbor called animal control and two of the kittens were picked up and killed the same day. So I had to build the room to keep them safe. 3 more feral cats showed up about 2 months ago, 1 was TNR, the second was trapped and spayed, but while I was taking care of her (she was pregnant), the third cat was found dead on another's neighbor backyard; I suspect poisoned). So now, I would like to keep her and place her in the room with the other 2 feral cats.
The situation is - Mom is a true feral, she is nice but does not allow me to touch her. Baby (not a baby anymore he is 1 and 1/2 years old now) is a feral but he allows me to touch him, he still is afraid of strangers. New cat (about 8 to 12 months), I believe she has been around humans but abandoned. She is sweet and lets me touch her, this is too soon for a feral.
I need help on how to introduce them properly to avoid fights. I feel the room is big enough, it has lots of shelves, cat tree, beds, little houses, toys, AC, heater, etc. My husband wants to build an enclosure inside the room (3' x 4' X 3') to keep her (new cat) inside for few weeks until everyone feels at ease........ I think this is a good idea, but need help. Right now she is in a huge dog crate, but it is a challenge to keep her inside.
Unfortunately, I am a dog person and have 7 in the house. So obviously, taking "the new cat" inside our house is out of the question.

Thanks in advance for any help.

Lyda