I'm sure you could substitute the Aida (if it's not quite the color you want it to be, just dye it using tea)
If you have a stash of DMC floss, just lay them out next to the pattern to pick what looks best.
As far as the whisper floss, that is what makes the hair fuzzy. (I have used it for Santa beards). If you still would rather not use it, do a Turkey stitch (google it, there is a great you tube video showing how to do it) for the manes & head tuffs.
You can make whatever changes you want, & it will still come out great

Like Nita said. Make any changes you want. The supply list is merely suggestions. The is no law that says you cannot make changes. I almost always change out floss colors and fabrics depending on my mood and what I have in my stash,

Nancye!
I can just see you with green giraffes and perhaps purple hippos! I also change things too, but I suspect neither of us go far off into left field! Not that I haven't had those thoughts!
When I see a familiar/popular design that is a bit staid, my brain kicks into its silly mode...like Prince Charles with an "A" on his shirt, or perhaps animals with fur/hides from Barnum and Bailey, or maybe get one of the popular Dimensions or Bucilla charts, not a kit, and mess it all up with our floss from our huge stashes. We could do "round robin" & include all!

SU - I would never, ever stitch a purple hippo. Purple is for cows!

However, I have two hippo patterns in my stash that both suggest purple!

Nancye...
My mom was a big Ogden Nash fan, so I say it is an inherited gene! Mom had two porcelain cows, a mom and a baby, in purple, of course! My brother and I saved six months for her for Mothers' Day. She loved them so much. I guess they were just another thing my older brother and wife took. They lived closer to mom. Ah well.