5? week old THROWN over fence as dog food

in my girlfriends yard. TG she was there as she has 2 Australian Sheep Dogs that thought special treats were coming as desert. A sibling escaped, I'll try trapping it in the evening.
So baby smells like it was rolled in Karo and Dog Food and looks like it too. It has one of the worst flea infestations I've ever seen. I carried it into my downstairs bath, fed it wet (it ate well) and watered it, bathed it to relieve matting (poor thing could hardly move it's limbs).
In addition to fleas it definately has an upper resp and malnutrition. I have a breeeder coming in the AM to administer coat care and will vet it in the afternoon.
I don't know what to do with it though if I can't rehome it. My colony is stable with 2 adult male and 1 adult female fixed cats. This is the first year I don't have kittens. I only have an acre and my cats are very territorial. This thing is smaller than my hand when open, I can't put it out. BUT I don't have any ferals, all mine are strays that seek attention when I'm outside. Maybe they will accept it if I take it slow with them?

What a cruel thing to do. People are crazy sometimes.
Are you planning on keeping them in or out? Them as in if you find the sibling?

Poor baby. I hate people sometimes. Anyway, thing to do is to KEEP
him/her, socialize with your indoor kitties and when he/she is
BIG enough, about 3 months you can let him/her outside.

During the time you make him/her social with your kitties,
you can slowly expose to outdoor kitties too. Eventually
both will get along, and eventuallty you can let him/her
be outdoor only....

Just make sure both groups get chances to smell each other.

There are other ways to do this too, putting him/her in cage out
side, so she / he gets time outside, the others can sniff him/her
but can't harm, and they all get used to him/her.

You can do this while you are outside gardening or loving
the other cats. This way they all get exposure.

But for now, this one is too young by far to be out.

Oh, and I forgot to add, some of the strays may take him/her
on as a buddy... cats do make friends sometimes and befriend
smaller cats...

Oh, poor little babies! I can't believe someone would do such a thing! I'm glad you rescued this precious little kitten. I hope you can find the other one before something worse happens to it.

Small wonder why I like animals better than people sometimes!!!

You've gotten excellent advice here. Good luck to you and your tiny rescues!