Snoopy went to the vet! * HELP !!!!!*

[Maya & Inka's mommy]

Some time ago, I posted about one of my cats peeing on the floor, in boxes, on clothing, etc.... ; up till yesterday I didn't know who did this. But then I saw little Snoopy peeing on a freshly washed catbed!!! Darn, I washed it that morning because I found cat-pee on it . And now Snoopy was "christening" it again !
I was glad though that I saw her doing it, now I know who is the culprit!!
I collected as much as possible of her pee in a glass, and brought it to the vet.
Today I went to the vet again, taking Snoopy with me. She has an infection and needs antibiotics; there are also chrystals in her pee . This is all new to me; is there anything I can do to prevent more problems? The vet advised to give her special food, but that will be almost impossible. Snoopy is our "vacuumer"; meaning: she eats all the leftovers of the other cats... . Can anybody give advise here?
As Snoopy is very difficult to pill, I ordered a pillshooter, and also the pillbags someone posted about lately. Let's hope she will let me give her the pills.... .

Little Snoopy was not very well behaving at the vets . She was mad when the vet tried to examine her belly, and she even bit her when she tried to clip her nails! Naughty little monkey!!!