On paper (or the computer screen) I'd have to say that this would be a difficult pairing. Sag and Aries may be too much fire for water signs Cancer and Pisces, and vice versa. She will probably be too active, selfish/ego-oriented, independent and tactless; he is likely to be too clingy/dependent, sensitive, emotional, mysterious and passive. Sun is your outer self, the person you intrinsically are; moon is inner self -- emotions, sub-conscious. In both of these areas, there will be fire/water conflict, or at the least, misunderstandings due to completely different ways of feeling, and of expressing said feelings, even energy levels and activities each enjoys are not likely to gel.
Having said that, there is far more going on in the chart, although Sun and Moon do play a big part in relationships. There may be areas in which you are perfect for each other -- the things you enjoy talking about, the way you communicate about your feelings, etc etc, while Cancer may enjoy letting Sag take the lead. There may even be things in your chart that incline you to enjoy, and deal well with, chaos and conflict!
I won't say don't go there, but I will say proceed with caution. In particular, be wary of giving too much to Sag, who may not think to give much in return, nor be able to identify or understand your needs.
I recommend you check out the free horoscopes on www.astro.com which will outline areas of difficulty as well as areas of promise. You will need times and places of birth, though.