Flea allergies - and/or ringworm?

I took a cat home on Sunday which someone found and brought to the vet
at PetsMart. She was pretty much completely covered in scabs from what
I thought was a flea-bite allergy. The vet at PetsMart looked her over
and said she may also have ringworm but it seemed to me like the vet
was more interested in listing everything that could be done to enlist
the 'prospective new client' into signing a Banfield Wellness Plan.
Their Wood's lamp didn't work, so short of sending off a skin-scraping
she couldn't even attempt to look further into the possibility of
ringworm.

Short of pictures, here's what she 'looks' like: Her stomach and
basically all her skin below her waist are scabbed up and there's only
a thin layer of hair left. She's got more fur and less scabs on her
back and front legs, with lots of scabs on top of her head. She's
short-haired.

Since I treated her with CapStar and Frontline on Sunday, she has not
been scratching herself any longer (though I have, I have managed to
avoid ringworm for the past three years of rescuing and I'm not
interested in getting it now!). She does groom herself a lot, but it
seems like she's just pulling off the scabs.

Any helpful thoughts?