Shih-Tzu companion questions?

Hiya! I have a 10 mnth. old male shihtzu puppy, he's very happy-go-lucky but doesn't like strangers... anyway, whenever we pass by other dogs while walking he tries to play with them and often I feel as if he's lonely playing by himself with his toys (I play with him of course, but I can't always). I'm trying to choose between getting a beagle or a chihuahua. Cujo my shih-tzu is a fairly large for a shihtzu so a getting a beagle shouldn't be too dangerous for them to play together... but on the other hand a chihuahua is slightly smaller than a shih-tzu. I have two different breeders in mind, both litters are around 6 months. Please help me decide whether Cujo would be better suited with a beagle or chihuahua playmate. Also, I want it to be a boy... would there be domination problems? Please, please help! Thanks :-D