If Gribble almost accidentally blew his foot off with one of the martian blasters just from trying to destroy some trash with it, then there's no question that Milo should have lost his foot when he got shot in the foot with one of them.
Milo's weight was less on Mars than Earth, which would be correct. However, when Gribble and Ki are on Earth, their weight should be higher than on Mars - by a factor of approximately three. This would have made it impossible for them to walk or really move around much at all.
Milo's Mom told him that cats are not supposed to eat vegetables. Cats can actually eat some vegetables like cooked/steamed broccoli and some others.