Is it at all possible to make Unity grass interactive, such as the grass being "pressed down" when you walk on top of it? Or would I just have to make a generic grass plane standing out of the ground and animate it?
Not entirely sure about Unity, but we used to do this with SpeedTree directly:
Use a wind object right next to the player's legs pushing outwards. For Unity, do something like the helicopter example from below
Let me know if this works. I'm curious as to whether they're using the same system for grass.
You could make it interactive, but interactive grass is quite costly on CPU power.
One way to do it would be to have colliders attached to the feet of your character, so when a collider hits the grass, an animation plays of some kind. Basically what you're describing.
Could anyone with better experience than me answer this?