Definitely not. Being seme would go counter to his personality
Agreed. :) But I think that Ryu is generally pretty mellow and shy, he gets aggressive only when his friends are in trouble or he's attacked himself. I would say that Shima is more pro-active. When they talk to each other, it is Shima who has more guts to say or initiate some things. That's why I believe that he is the one in a controlling position, even though he is shorter and less violent. And with other people he is also hard to push around. He knows what he wants and fights for it with paying no respect to what others think about it.
I think this is so annoying... Ryuu-San should be the seme ( ̄ε(# ̄)Σ