Modern reptiles aren't normally found in a group. They're usually quite solitary creatures that only manage to come together from the urge to reproduce. If you must house reptiles together, house lizards of the same species and try to choose those that are close in size. Mature male lizards and snakes should never be housed together, due to the very real risk of fighting. You may think your reptile is lonely but actually, he is quite content with his own personal castle.
For more information, please read the article Can Reptiles be Social?