Anubias and java fern can be put in the gravel. But she NEEDS to make sure the rhizome (the stem-like thing that the leaves grow from) is above the gravel. As long as it is just the roots in the gravel, it's perfectly fine. I have my java fern planted in the gravel in my betta tanks. They're doing just fine.
For water changes, 40% weekly is a good starting point. Even with live plants it's better to start off large and back it down rather than start small and increase only after you see a problem.
I wouldn't do cories or any other bottom fish if she just has gravel. Not all gravel is soft enough for them. Plus, platies are pretty all over the place. Also, the ones with the deformed spines will have a shorter life span. Any fish that has been modified from the wild form like goldfish or balloon mollies will have a reduced life span.