Cat with Chronic Diarrhea, test for infectious diseases?

I need opinions please.

I brought home 2 feral cats from a shelter about 4 months ago. They were presumed to be sisters. The one cat, Lita, has had loose stool and diarrhea since she has been here. I gave her a couple weeks to see if it was stress related. After that I had a fecal exam done and nothing showed. They suggested it could be stress or food allergies.

I switched the cats to Wellness, then to Natural Balance and Burns but nothing helped so I don’t believe its diet related.

I took another sample in yesterday to do a fecal smear and they found nothing again.

They want me to test her for FeLV, FIP and FIV.

I have read those tests are not 100% accurate. I also see there are many symptoms that come with these infectious diseases, none of which Lita has, just chronic diarrhea with blood.

Any thoughts on what this could be and if the test is a good idea?

I am really upset as I took this girl in to give her a better life but at the same time I can’t have her infecting my other 5 cats. She stays upstairs with my male cat and her sister and they share the same litterboxes so I think my other 3 kitties are safe but Lita, Lily and Sully are very close and play hard so there is high risk that they have been exposed to it through contact and sharing litterboxes.

If there is blood in her stool, she needs antibiotics.
Since you have other cats, it may be a good idea to have her checked for those viruses because although they don't transmit easily (like a cold), it does mean you'll have to take precautions. This could be an opportunistic infection due to damaged immunity and must be treated. I'm just speculating here so don't worry until you find out. Start with the treatment and go from there.

I have talked to the vet and the other test besides those for infectious diseases is for giardia, which has to be sent out. It would be around $50.

We have decided to treat the 3 cats with panacur to cover worms/parasites and giardia rather then test as it's more cost effective and will get any other worms/parasites that might not have showed up in the fecal smear. If that does not work then test for FIP, FIV, FeLV.

Sound normal? Because she is feral I am trying to do everything first that DOES NOT include a visit to the vet. It would be terribly tramatic for her.