welcome to YT and congrats on your new baby!
about the litter box problem........get rid of the cat!
just joking ...........I have never had to deal with cat problems due to the fact that I am allergic. sorry no help there.
have your vet go over her real well and they will suggest a heart worm preventive along with flea preventive. we use interceptor and frontline plus.
the dandruff sounds like it might be dry skin from the flea treatment. It is awful harsh. I would recommend the mane and tail shampoo and conditioner. you can get a big bottle of both for about $5.00 each at walmart. it is the best cheap stuff out there, in my opinion. I keep some around in case I run out of the better stuff.
On those areas that are red from the scratching you can apply some neosporin. It will help heal almost over night. when Ahia gets a bug bite it sure helps her.
One thing you have to remember to do when shampooing is to rinse and rinse until you think you have it rinsed good enough then rinse again.or she will get the itchies reguardless of the type of shampoo.
I would get her on a good natural dog food. we have switched to natural balance. I got the allergy formula because of Ahia's allergies that make her itch. I think the corn filler in science diet was her problem.
I bet Sugar is just as happy with you as you are with her.