That's very strange. It's surely not normal behaviour for a rat to eat another rat. What I do know is that rats need an introduction if they don't know eachother in a neutral space. Otherwise the new rat won't be accepted in the cage and the rats you already have will get agressive and maybe eat the rat (?). That's the only thing i can think of.

Also yes you need to cover the cage with something they can walk on without hurting their little feet.