For the sake of arguement, let's just stick Heero Yuy at the peak point, that of Spirit. Why? Well, Quatre did call him the "Soul of Outer Space", and that seems like an big hint. Even without further evidence (which there is, I just haven't got on hand), we'll leave him there.
Wufei seems like an obvious placement in Fire. Not only is Nataku often shown standing in/around/near fire (comes from having a flamethrower...), Wufei is depicted as having a true Fire disposition. He is aggressive, stubborn, hot-headed, short-tempered, occasionally rash, and fights for those who cannot fight for themselves with zeal/passion, which fits the description.
Quatre seems like another obvious placement in Water. Yeah, I know... he comes from a desert. Believe it or not, deserts- including the Sahara and Gobi- are locations ruled by the element of Water. Also, Quatre has a the personality of a Water element/sign: sysmpathetic/empathetic, sensitive, artistic, emotional. Quatre to a T.
Duo and Trowa are a little harder to place... but still there. Trowa under Air and Duo as Earth. Trowa as Air is both appropiate and amusing, as Air is the element of communication... Trowa's got Mark Twain's grip on communication: "Drawing on my fine command of language, I said nothing." He has a very Airy build, and moves with the grace and assuredness the element dictates.
This leaves Duo as Earth. Earth signs are stubborn, relaxed, and into the finer things in life- like having fun.
There are other possible arrangements around the pentagram, and I will probably try to get a few more. (One arrangement could be Heero as Air, Quatre as Earth, Duo as Water, Wufei as Fire, and Trowa as Spirit... how'd I get that? You figure it out for now.) Any suggestions are welcomed.