Some states, California is one I think, require water heaters to be strapped to the wall to prevent tipping in case of earthquakes. As far as stability under normal conditions, you would be hard pressed to tip a full tank over. If it were to tip, I don't think a ridged pipe connection, which would be at the base of the tank, would help very much. Further, I believe a solid gas line connection to any appliance would be subject to breaking in case of any movement. You couldn't even have a gas stove that was recessed in a counter because you couldn't connect the pipe with the stove in position. Flex gas lines are pretty standard fare with gas appliances.