Let's say I am a druid that shifts and gets an animal with the grab feat, but I have also have the improved grapple feat. Does Garb stack with Improved Grapple?
Yes. They are separate abilities and nothing suggest that they don't stack.
(unless there is some FAQ/errata I am not aware of where designers stated they that wasn't their intent for the abilities to stack).
They should stack. If you don't want to spend all of your feats on this just take a level or two of Monk. The Wisdom bonus to AC can be nice too.