Total Recommendations: 4
Both series have an agressive female character(Nyaruko-san for Haiyore! Nyaruko-san and Ryoko of Tenchi Muyo!) who lusts after the male lead and often makes unwanted advances towards their respective paramour. Both series are harems, although Tenchi seems to show more caring towards his than Mahiro does.
Both are created by Yuzo Takada and the main heroines are voiced by Megumi Hayashibara. Blue Seed also deals with the supernatural with an underlying horror theme, and both shows have a romance develop between the leads.
Both shows take place admist a dystopian society where it's man vs. machine, the female characters are fierce fighters, and share Kenichi Sonoda as the character designer. The only difference is that Gall Force: Chikyuu Shou takes place in the late 21st century in Australia with some scenes on Mars.
Even though Rumiko Takahashi only did the character designs for Moeyo Ken, this series has a similar feel to Ranma 1/2 with the comedy, the characters, and the romance. For example, Yuuko Kondo has a personality akin to girl-type Ranma Saotome and the young waitress girl Sayoko Yamazaki has both the appearance and personality like Kasumi Tendo.