I wondered how securely they stacked on one another and how easily they could be tipped off of there unless there's some kind of groove that locks them in place somewhat. And also because I'm looking at a small unit in front of a larger unit, they're not linked together in any way, correct? So basically if you had three of the taller ones and then three of the smaller ones in front you would just have to move the stack of smaller ones to get to the big ones? Three quick answers would be appreciated. Thank you.