The ruling planet for Picses is Neptune and its element is water, just as Sailor Neptune controls the seas. Aquarius' ruling planet is Uranus and its element is air, just as Sailor Uranus is tied so closely to the wind.
Takeuchi considers Takarazuka to be the "maximum level of feminine emancipation" and decided to use the Revue as a way to explain to children how two women could be in love with each other. As Takarazuka is popular in Japan, it would be a lot easier to understand when using this as a reference. Haruka is otokoyaku, so she naturally would fall in love with Michiru, the musumeyaku.
Haruka's blood type is O which means that she is wild, active, creative, passionate and strong. It also means that she is selfish, irresponsible, unforgiving and erratic. This seems to suit Haruka's personality very well, as she is extremely active, strong and passionate in her mission but she can also be rather selfish at times. While I would not necessarily say Haruka is erratic, she can be unforgiving regarding anyone who hurts those she protects.
Michiru's blood type is O, meaning that she is confident, self-determined, optimistic, strong-willed and intuitive. Negatively, it means she's self-centered, cold, doubtful, unpredictable and a workaholic. While Michiru is certainly strong-willed, confident and intuitive, I'm not sure she's self-centered or unpredictable. She can be cold as Sailor Neptune when she has to be.
Sources
Interview with Takeuchi Naoko - http://www.kicie.net/realm/naoko.htm
Wikipedia information on Takarazuka Revue - http://en.wikipedia.org/wiki/Takarazuka_Revue
"Blood types in Japanese culture" - http://en.wikipedia.org/wiki/Blood_types_in_Japanese_culture