The crab boss is Clawgrip from World 5.
I wrote out all the signs here (including the ideas you originally wanted to use) just to keep my thoughts straight while I helped brainstorm.
Aries - Wario (an ambitious leader who has built a gaming empire of his own, independent of Mario himself (wario world/ware) where he is a main character), or any other character who functions as a leader in the series would work well here
Taurus - Thwomp! (Made of stone and almost impossible to budge. If you get in its way, watch out!
)
Gemini - Mario/Luigi, or alternatively, any of the characters which come in palette-swapped colors (like the Toads from New Super Mario Bros Wii, Koopa Troopas, Yoshis, etc)
Cancer - A Spiny or a Koopa Troopa? (to represent Cancer's defensive nature and its love of home, since it's carried on the back of the critter?) or a Shy Guy (tough to get out of its shell?)
Leo - Maybe the Angry Sun from the Super Mario 3 desert level (since Leo is ruled by the Sun)? Any character/enemy that is gold-colored may also work, since I think Leo's color is golden yellow. (Princess Daisy wears yellow, so she might work?)
Virgo - Princess Peach
Libra - The idea of Bowser in his boss fight vehicle is an awesome idea for this, Lemming!
Scorpio - Bomb-Omb (to represent Scorpio's potentially deadly temper?) or a Boo (since Scorp will get you when you aren't looking?)
Sagittarius - Red Yoshi (since he throws fireballs and Sag is an archer and a fire sign) ridden by another human character of your choice (to incorporate the centaur half man/half beast aspect of the sign)?
Capricorn - A Hammer Brother? (Since Capricorns are hard workers, a hammer is a tool, and very practical to boot since Cap is an earth sign?)
Aquarius - Frog Mario
Pisces - Cheep Cheeps (I believe that's what the fish are called)
Hope this at least helps the brainstorming a bit! I hope you will post pictures of the finished project when you make it.
