There are infectious causes of gingivitis, and stomatitis which can be passed on from one cat to the others in a multi-cat household, like yours. The most common causes are Bartonella disease, Calicivirus, feline infectious peritonitis, Felv/Fiv, and feline herpes virus.
Bartonella (cat scratch fever) is treated by the antibiotic Azithromax. Herpes outbreaks can be controlled by using Lysine supplementation.
There is also a non-infectious cause of stomatitis called Lymphocytic, Plasmacytic Stomatitis. This is an autoimmune disease which can be treated by antibiotics such as Azithromax or Clindamycin and Steroids, such as prednislolone. The mouth lesions of Lymphocytic, Plasmacytic Stomatitis can resemble mouth cancer.
It would require extensive diagnostics to try to discover the cause of your cat's gum problems.
If the Clindamycin is ineffective, you can try Azithromax. Brushing and flossing your cats teeth, and using Hill's Feline T/D (tartar diet) in place of some of your cats regular food would also help.
It might be a good idea to have the mouth lesion biopsied. If the results of the biopsy come back as Lymphocytic/Plasmacytic Stomatitis ask your vet about using prednisolone and antibiotics.
Oh thank you so very very much for the good info!!! I intend to make the vet biopsy her mouth if this Clindamycin does not help. As I stated before, none of them are FIV possitive, but I do know their immune systems are not up to par. I also thought about Calici virus in the beginning, but since the first one to come down with this problem has never been outside, then it doesn't seem likely. The other things you mentioned I will look straight into!
Again thank yoooooooouuuuuu!!!!